Field of the invention The present invention pertains generally to the field of image processing and more precisely to the coding and to the decoding of digital images and of sequences of digital images. The invention can be applied in particular, but not exclusively, to the video coding implemented in current AVC and HEVC video coders and their extensions (MVC, 3D-AVC, MV-HEVC, 3D-HEVC, etc), as well as to the corresponding decoding. Background of the invention Current video coders (MPEG, H.264, HEVC, etc) use a block-wise representation of the images to be coded. The images are subdivided into blocks of square or rectangular shape, which are liable to be subdivided in their turn in a recursive manner. For at least one block considered from among the various blocks obtained, a prediction of pixels of the block considered is implemented with respect to prediction pixels which belong either to the same image (Intra prediction), or to one or more previous images of a sequence of images (Inter prediction) which have already been decoded. Such previous images are conventionally called reference images and are preserved in memory either at the coder or at the decoder. In the course of such a prediction, a set of data is calculated by subtracting the pixels of the block considered, from the prediction pixels. The coefficients of the calculated data set are then quantized after a possible mathematical transformation, for example of discrete cosine transform type (DCT), and then coded by an entropy coder. The coded data are written into a data signal intended to be transmitted to a decoder. Said data signal comprises in particular: the type of prediction (Intra prediction, Inter prediction, default prediction carrying out a prediction for which no information is transmitted to the decoder (in English "skip")); - the mode of prediction (direction of prediction, reference image component, etc); - the type of subdivision into sub-blocks; - the type of transform, for example 4x4 DCT, 8x8 DCT, etc...; - the values of pixels, the values of transformed coefficients, amplitudes, signs of quantized coefficients of the pixels contained in the block or the sub-block considered. Once the data signal has been received by the decoder, the decoding is done image by image, and for each image, block by block. For each block, the corresponding elements of the data signal are read. The inverse quantization and the inverse transformation of the coefficients of the blocks are performed so as to produce a decoded block of residual data. Next, the prediction of the block is calculated and the block is reconstructed by adding the prediction to the decoded block of residual data. In a manner known per se, the quantization of the coefficients of the calculated block of residual data can be of scalar or vector type. The scalar quantization uses a quantization step which is determined on coding on the basis of a parameter called QP (English abbreviation standing for "Quantization Parameter"). The vector quantization consists on coding: - in comparing the calculated set of residual data with a plurality of quantization vectors grouped together in at least one dictionary, - in selecting one of the quantization vectors according to a predetermined coding performance criterion, such as for example the bitrate-distortion compromise well known to the person skilled in the art, - in coding an index associated with said selected quantization vector. The coded index is then transmitted in the data signal destined for the decoder which contains the same dictionary or dictionaries as the coder and which applies the inverse vector quantization by decoding said transmitted index, and then by determining the quantization vector associated with said decoded index. Having regard to the fact that the quantization vectors form part of a predetermined dictionary, the coding of the current block by vector quantization may turn out to be poorly suited to the statistics of the video signal. Ultimately, this results in unsatisfactory compression performance. Object and summary of the invention One of the aims of the invention is to remedy drawbacks of the aforementioned prior art. For this purpose, a subject of the present invention relates to a method for coding at least one image split into blocks, implementing, for a current block to be coded of the image: - a prediction of the current block in accordance with a mode of prediction selected from among a plurality of predetermined modes of prediction, - an obtaining of a predictor block on completion of the prediction, - a calculation of a first set of data representative of a difference between the predictor block obtained and the current block, - a comparison of the first calculated set of data with a plurality of quantization vectors, - a selection of one of the quantization vectors according to a predetermined coding performance criterion, - a coding of an index associated with the selected quantization vector. The coding method according to the invention is noteworthy in that it comprises: - a calculation of a second set of data representative of a difference between the first calculated set of data and the selected quantization vector, - a coding of the second calculated set of data in the course of which at least one of the quantization vectors is modified as a function of the data of the second calculated set of data. Such a provision advantageously makes it possible to aggregate: - the benefit derived from utilizing the correlations between the first current set of residual data and the quantization vectors of a dictionary, - and the benefit derived from an updating of at least one of said quantization vectors, said updating making it possible to adapt, in the course of time, to the modifications of the signal of coded data. The prediction of the current block is thus both more precise and more suited to the current video context, the advantage of this being to significantly improve the performance in respect of compression of the signal of coded data to be transmitted to the decoder. In a particular embodiment, the modification of one of the quantization vectors is implemented only if the data of the second calculated set of data fulfill a predetermined criterion. Such a provision makes it possible for the modification of one of the quantization vectors to be performed only when this modification is substantial, thereby avoiding needless calculations. In another particular embodiment, the modification of one of the quantization vectors is implemented with the aid of a parameter whose value depends on the size of the image to be coded. Such a provision makes it possible to adapt to the size of the data at the disposal of the coder. Thus, if the image is small, the modifications of the dictionary must be of large amplitude, since few data are at the coder's disposal in order to capture the statistics of the video signal. On the other hand, if the image is large, it is possible to favor an updating of low amplitude, which is more resistant to irrelevant statistical variations of the video signal, since much more data are at the coder's disposal in order to adapt to these variations. In yet another particular embodiment, if the current image is of Intra type, the quantization vectors are each initialized to predetermined values. Such a provision makes it possible to keep the coding characteristic independent of the other images which is specific to the Intra images, even if the invention proposes an updating of the dictionary of quantization vectors which evolves in tandem with the processing of the images of a considered sequence of images. Indeed, it is generally desirable that the images of Intra type be coded and then decoded independently of any other image of the video stream, so as to be able to decode a video in the course of transmission (for example, to change channel on a television receiver, in the case of the broadcasting of an audiovisual stream). Thus, when an Intra image of this sequence must be coded, it cannot be coded with respect to a quantization vector of a dictionary which has been updated for an image previous to this Intra image. Consequently, the invention advantageously proposes initializing the quantization vectors of the dictionary to predetermined respective values so as to render the coding of the Intra Image independent of any other image of the sequence. The various aforementioned embodiments or characteristics of realization can be added independently or in combination with one another, to the operations implemented in the course of the coding method such as defined hereinabove. Correlatively, the invention relates to a device for coding at least one image split into blocks, comprising, for a current block to be coded of said image: - a prediction module for predicting the current block in accordance with a mode of prediction selected from among a plurality of predetermined modes of prediction, delivering a predictor block, - a calculation module for calculating a first set of data representative of a difference between the predictor block obtained and the current block, - a comparison module for comparing the first calculated set of data with a plurality of quantization vectors, - a selection module for selecting one of the quantization vectors according to a predetermined coding performance criterion, - a first coding module for coding an index associated with the selected quantization vector. Such a coding device is noteworthy in that: - the calculation module is activated to calculate a second set of data representative of a difference between the first calculated set of data and the selected quantization vector, and in that it comprises a second coding module which is activated to code the second calculated set of data, at least one of the quantization vectors being modified as a function of the data of the second calculated set of data. In a corresponding manner, the invention also relates to a method for decoding a data signal representative of at least one image split into blocks, implementing, for a current block to be decoded: - a decoding: • of an index associated with a quantization vector which belongs to a set of quantization vectors, • and of prediction data in respect of the current block to be decoded, - a decoding of the index, - a determination of the quantization vector associated with the decoded index, - an obtaining of a predictor block on the basis of the decoded prediction data. Such a decoding method is noteworthy in that it comprises: - a determination, in the data signal, of data relating to the current block to be decoded, - a decoding of the data relating to the current block to be decoded, in the course of which at least one of the quantization vectors is modified as a function of the decoded data, - a reconstruction of the current block on the basis of the decoded data, of the quantization vector determined and of the predictor block obtained. The data relating to the current block to be decoded are typically the values of the pixels of this block making it possible to reconstruct this block or values of transformed coefficients making it possible to reconstruct this block. In a particular embodiment, the modification of one of the quantization vectors is implemented only if the decoded data fulfill a predetermined criterion. In another particular embodiment, the modification of one of the quantization vectors is implemented with the aid of a parameter whose value depends on the size of the image to be decoded. In yet another particular embodiment, if the current image is of Intra type, the quantization vectors are each initialized to predetermined values. The various aforementioned embodiments or characteristics of embodiment can be added independently or in combination with one another to the steps of the decoding method such as defined hereinabove. Correlatively, the invention relates to a device for decoding a data signal representative of at least one image split into blocks, comprising, for a current block to be decoded: - a first decoding module for decoding: • an index associated with a quantization vector which belongs to a set of quantization vectors, and prediction data in respect of the current block to be decoded, - a calculation module for determining the quantization vector associated with the decoded index, - a prediction module for obtaining a predictor block on the basis of the decoded prediction data. Such a decoding device is noteworthy in that it comprises: - a second decoding module which is activated to decode data determined in the data signal and relating to the current block to be decoded, at least one of the quantization vectors being modified as a function of the decoded data, - a reconstruction module for reconstructing the current block on the basis of the decoded data, of the quantization vector determined and of the predictor block obtained. The invention further relates to a computer program comprising instructions for implementing the coding method and/or the decoding method according to the invention, when it is executed on a computer. Such a program can use any programming language, and be in the form of source code, object code, or of code intermediate between source code and object code, such as in a partially compiled form, or in any other desirable form. Another subject of the invention also envisages a recording medium readable by a computer, and comprising computer program instructions, such as mentioned hereinabove. The recording medium can be any entity or device capable of storing the program. For example, the medium can comprise a storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or else a magnetic recording means, or else a digital recording means such as for example a USB key or a hard disk. Moreover, such a recording medium can be a transmissible medium such as an electrical or optical signal, which can be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can be in particular downloaded from a network of Internet type. Alternatively, such a recording medium can be an integrated circuit in which the program is incorporated, the circuit being adapted to execute the coding and/or decoding method according to the invention or to be used in its and/or their execution. Brief description of the drawings Other characteristics and advantages will become apparent on reading a preferred embodiment described with reference to the figures in which: - Figure 1 represents the steps of the coding method according to one embodiment of the invention, - Figure 2 represents a coding device implementing the steps of the coding method of figure 1, - Figure 3 represents a decoding device according to one embodiment of the invention, - Figure 4 represents the steps of the decoding method which are implemented in the decoding device of figure 3. Detailed description of the coding part An embodiment of the invention will now be described, in which the coding method according to the invention is used to code an image or a sequence of images according to a binary stream close to that which is obtained by a coding implemented in a coder in accordance with any one of the current or forthcoming video coding standards. In this embodiment, the coding method according to the invention is for example implemented in a software or hardware manner by modifications of such a coder. The coding method according to the invention is represented in the form of an algorithm comprising steps C1 to C15 such as represented in figure 1. According to this embodiment, the coding method according to the invention is implemented in a coding device CO represented in figure 2. As illustrated in figure 2, such a coding device comprises a memory MEM_CO comprising a buffer memory TAMP_CO, a processing unit UT_CO equipped for example with a microprocessor uP and driven by a computer program PG_CO which implements the coding method according to the invention. On initialization, the code instructions of the computer program PG_CO are for example loaded into a RAM memory MR_CO before being executed by the processor of the processing unit UT_CO. The coding method represented in figure 1 applies to any current image ICj which is fixed or else which forms part of a sequence of L images ICi, ..., ICj ICL(11. According to a preferred embodiment, said blocks are 4x4 or 8x8 pixels in size. As a function of the size of the image which is not necessarily a multiple of the size of the blocks, the last blocks on the left and the last blocks at the bottom might not be square. In an alternative embodiment, the blocks may be for example of rectangular size and/or not aligned with one another. Each block may moreover be itself divided into sub-blocks which are themselves subdividable. In the course of a step C2 represented in figure 1, the coder CO selects as current block a first block to be coded Bu of the image IQ, such as for example the first block B1. In the course of a step C3 represented in figure 1, there is undertaken the prediction of the current block Bu by known techniques of Intra and/or Inter prediction. For this purpose, the block Bu is predicted with respect to at least one predictor block in accordance with a mode of prediction selected from among a plurality of predetermined modes of prediction MPo, MP1,..., MPV,..., MPQ where 0, are constructed with a view to the updating, in the following manner: where: - alpha is a predetermined parameter, for example equal to 0.1, - and f(n) is a value which depends on the distance between the index n and the index opt. For example, in the preferred embodiment: The vectors Z0pt+n with n ranging from -5 to +5 are therefore calculated, and respectively replace the vectors Vopt+n in the dictionary CBKopt. The set of current dictionaries CBK1, CBK2,..., CBKw is therefore thus re-updated. In an alternative embodiment, the parameters of the updating can be different according to the size of the image. Indeed, if the image is small, a fast learning of the statistics of the current image IQ is necessary. For example: - for an image size belonging to a single-definition SD video (the English abbreviation standing for "Standard Definition"), i.e. fewer than 720 pixels high and fewer than 1280 pixels wide, a parameter alpha equal to 0.3 is adopted; - for an image size belonging to a High-Definition HD video, i.e. between 720 and 1080 pixels high and between 1280 and 1920 pixels wide, or much greater than 1280x1920 pixels, a parameter alpha equal to 0.2 is adopted; - for an image size greater than the size of an HD image, a parameter equal to 0.1 is adopted. It is of course possible to re-update the dictionaries in other ways. For example, it is possible to apply the same type of quantization vector updating as in the preferred mode. However, the updating, instead of being applied to the neighbor vectors of the quantization vector Vopt in the dictionary CBKopt, is applied to the vectors close to Vopt+BSDru in the sense of the distortion. As a variant, the updating is applied to vectors close to Vopt+BSDru not only in the dictionary CBKopt, but in the set of current dictionaries CBKi,CBK2,..., CBKw. Subsequent to the aforementioned step C12, there is undertaken, in the course of a step C13 represented in figure 1, the selection of the following block of the current image IQ. Next, the block coding steps described hereinabove are again implemented for this following block. If on completion of the aforementioned step C11, the updating criterion is not fulfilled, there is undertaken, in the course of the aforementioned step C13, the selection of the following block of the current image IQ. Next, the block coding steps described hereinabove are again implemented for this following block. In the course of a step C14 represented in figure 1, the coder CO of figure 2 tests whether the current block which has been coded in accordance with the coding method described hereinabove is the last block of the current image IQ. If such is not the case, the aforementioned step C12 is implemented. If the current block is the last block of the current image IQ, in the course of a step C15 represented in figure 1, the coder CO of figure 2 tests whether or not the following current image IQ+i is an image of Intra type. In the case where the following current image IQ+i is of Intra type, before undertaking the coding of the blocks of this image in accordance with the coding method of figure 1, step C12 of updating the dictionaries is implemented. In the example represented, the quantization vectors of the dictionary CBKopt are then each initialized to a predetermined respective value. In the case where the following current image IQ+i is not of Intra type, there is directly undertaken the coding of the blocks of this image in accordance with the coding method of figure 1. The coding steps C1 to C15 which have just been described hereinabove are thereafter implemented for each of the blocks Bi, B2,..., Bu,..., Bs to be coded of the current image IQ considered, in a predetermined order which is for example the lexicographic order. Detailed description of the decoding part An embodiment of the invention will now be described, in which the decoding method according to the invention is used to decode a data signal or stream representative of an image or of a sequence of images which is able to be decoded by a decoder in accordance with any one of the current or forthcoming video decoding standards. In this embodiment, the decoding method according to the invention is for example implemented in a software or hardware manner by modifications of such a decoder. The decoding method according to the invention is represented in the form of an algorithm comprising steps D1 to D15 such as represented in figure 4. According to this embodiment, the decoding method according to the invention is implemented in a decoding device or decoder DO represented in figure 3. As illustrated in figure 3, according to this embodiment of the invention, the decoder DO comprises a memory MEM_DO which itself comprises a buffer memory TAMP_DO, a processing unit UT_DO equipped for example with a microprocessor uP and driven by a computer program PG_DO which implements the decoding method according to the invention. On initialization, the code instructions of the computer program PG_DO are for example loaded into a RAM memory, denoted MR_DO, before being executed by the processor of the processing unit UT_DO. The decoding method represented in figure 4 is applied to a data signal or stream F representative of a current image IQ to be decoded which is fixed or which belongs to a sequence of images to be decoded. For this purpose, information representative of the current image IQ to be decoded is identified in the data signal F received at the decoder DO and such as delivered on completion of the coding method of figure 1. With reference to figure 4, in the course of a step D1, there is undertaken the identification in the signal F of the quantized residual blocks BSqi, BSq2,..., BSqu, ..., BSqs(1^u1. According to a preferred embodiment, said blocks to be decoded are 4x4 or 8x8 pixels in size. As a function of the size of the image which is not necessarily a multiple of the size of the blocks, the first blocks at the top, on the left of the image, and the last blocks at the bottom, on the right of the image, might not be square. In an alternative embodiment, the blocks may be for example of rectangular size and/or not aligned with one another. Each block to be decoded can moreover be itself divided into sub-blocks which are themselves subdividable. In the course of a step D2 represented in figure 4, the decoder DO of figure 3 selects as current block the first quantized block BSqu which contains quantized data which were coded in the course of sub-step C93 of figure 1. In the course of a step D3 represented in figure 4, there is undertaken a decoding of the information relating to the prediction of the current block Bu such as implemented on coding, in the course of step C3 of figure 1, and which were written into the data signal F. Such reconstruction information comprises the type of prediction (Inter or Intra) applied in step C3, and if appropriate, the mode of prediction selected, the index IBopt of the predictor block obtained BPopt obtained on completion of step C3, the type of partitioning of the current block Bu if the latter has been partitioned, the reference image index and the displacement vector that were used in the Inter mode of prediction. Such a decoding step D3 is implemented by the binary decoding module DB_DO represented in figure 3. In the course of a step D4 represented in figure 4, there is undertaken the predictive decoding of the current block to be decoded with the aid of the index IBopt of the predictor block BPopt which was decoded in the course of the aforementioned step D3. For this purpose, in a manner known per se, there is undertaken, in association with the decoded index of the predictor block BPopt, the selection, in the buffer memory TAMP_DO of the decoder DO of figure 3, of the corresponding predictor block BPopt, which figures among a plurality of candidate predictor blocks stored beforehand in the buffer memory TAMP_DO. Each of the candidate predictor blocks is a block of pixels which has already been decoded. Step D4 is implemented by an inverse prediction software module or processor PRED"1_DO such as represented in figure 3, which is driven by the microprocessor uP of the processing unit UTJDO. In the course of a step D5 represented in figure 4, there is undertaken a decoding of the index IVopt of the optimal quantization vector Vopt which was selected on completion of step C6 of figure 1. Such a decoding step D5 is implemented by the decoding module DB_DO of figure 3. In the course of a step D6 represented in figure 4, there is undertaken the determination of the optimal quantization vector Vopt associated with the decoded index IVopt. Step D6 is implemented by a calculation software module or processor CAL1_DO such as represented in figure 3, which is driven by the microprocessor uP of the processing unit UT_DO. In the course of a step D7 represented in figure 4, there is undertaken the selection of a dictionary of quantization vectors, denoted CBKopt, which contains the quantization vector Vopt selected in step D6. Such a dictionary belongs to a plurality of available dictionaries of quantization vectors, denoted CBKi, CBK2, CBKw. Such dictionaries are stored beforehand in the buffer memory TAMP_DO of the decoder DO, such as represented in figure 3. Step D7 is implemented by a calculation software module or processor CAL2_DO such as represented in figure 3, which is driven by the microprocessor uP of the processing unit UT_DO. In accordance with the invention, in a manner corresponding to the coding, such a selection is implemented as a function of one and/or of the other of the following elements: - the characteristics of the prediction (for example, the Intra mode chosen from among the 35 Intra modes of the HEVC standard), whose indices have been decoded in step D3, - the frequency characteristics of the predictor block BPopt whose index IBopt has been decoded in step D3, - the size of the current block Bu to be decoded, - characteristics of the current image IQ to be decoded, such as its size or its energy. In the preferred embodiment, W=70, that is to say that: - in the case of a current block Bu of size 4x4, there exists a different dictionary for each of the 35 Intra modes considered in the HEVC standard, - in the case of a current block Bu of size 8x8, there exists a different dictionary for each of the 35 Intra modes considered in the HEVC standard. In accordance with this preferred embodiment, the dictionary selected therefore depends both on the size of the current block Bu to be decoded and on the mode of prediction whose index has been decoded in step D3. In the course of a step D8 represented in figure 4, there is undertaken, in accordance with the invention, a decoding of the data of the quantized residual block BSqu. In the course of step D8, there is undertaken, in the course of a sub-step D81 represented in figure 4, a decoding of the current set of quantized coefficients BSqu. Such a decoding is for example an entropy decoding of CABAC type or else an entropy decoding of arithmetic or Huffman type. On completion of the aforementioned sub-step D81, a set BSDqu of digital information associated with the current set of quantized coefficients BSqu is obtained. Such a decoding sub-step D81 is implemented by an entropy decoding module MD_DO represented in figure 3, which is driven by the microprocessor uP of the processing unit UT_DO. In the course of step D8, there is undertaken, in the course of a sub-step D82 represented in figure 4, a dequantization of the digital information obtained subsequent to sub-step D81, according to a conventional dequantization operation which is the operation inverse to the quantization implemented during the quantization sub-step C92 of figure 1. A current set of dequantized coefficients BSDtu is then obtained on completion of sub-step D82. Such a dequantization sub-step is for example of scalar or vector type. Sub-step D82 is performed by means of a quantization software module or processor MQ"1_DO represented in figure 3, which is driven by the microprocessor uP of the processing unit UTJDO. In the course of step D8, there is undertaken, in the course of a sub-step D83 represented in figure 4, a transformation of the current set of dequantized coefficients BSDtu, such a transformation being an inverse direct transformation. This transformation is the operation inverse to the transformation performed in sub-step C91 of figure 1. On completion of sub-step D83, a current decoded residual block BSDru is obtained. Sub-step D83 is implemented by an inverse transformation software module or processor MT_1_DO, such as represented in figure 3, which is driven by the microprocessor uP of the processing unit UT_DO. The processor MT_1_DO is able to implement an inverse direct transformation such as for example an inverse discrete cosine transformation of DCT"1 type, an inverse discrete sine transformation of DST"1 type, an inverse discrete wavelet transformation of DWT1 type. In the course of a step D9 represented in figure 4, there is undertaken, in accordance with the invention, the reconstruction of the current block Bu by adding to the decoded residual block BSDru, obtained on completion of sub-step D83: - the optimal predictor block BPopt which was obtained on completion of the aforementioned step D4, - and the optimal quantization vector Vopt which was obtained on completion of the aforementioned step D6. On completion of step D9, a current decoded block BDU is obtained. Step D9 is implemented by a calculation software module or processor CAL3_DO represented in figure 3, which is driven by the microprocessor uP of the processing unit UT_DO. In the course of a step D10 represented in figure 4, said decoded block BDu is written in a decoded image IDj. Such a step is implemented by an image reconstruction software module or processor URI such as represented in figure 3, said module being driven by the microprocessor uP of the processing module UT_DO. In the course of a step D11 represented in figure 4, there is undertaken a test which consists in verifying whether a criterion for updating the dictionaries CBKi,CBK2,..., CBKw is or is not fulfilled. According to a first variant, such a criterion consists in comparing the number of non-zero coefficients in the decoded residual block BSDru with a predetermined threshold. For example, the updating criterion is considered to be fulfilled if the number of non-zero coefficients is greater than 3. According to a second variant, such a criterion consists in comparing the bitrate of the coding of the decoded residual block BSDru with a predetermined threshold. For example, the updating criterion is considered to be fulfilled if the bitrate of the coding of the decoded residual block BSDru is greater than 10 bits. Step D11 is implemented by a calculation software module or processor CAL4_DO such as represented in figure 3, which is driven by the microprocessor uP of the processing unit UT_DO. If the updating criterion is fulfilled, in the course of a step D12 represented in figure 4, there is undertaken an updating of at least one of the dictionaries CBKi,CBK2,...,CBKw. Step D12 is implemented by a calculation software module or processor CAL5_DO such as represented in figure 3, which is driven by the microprocessor uP of the processing unit UT_DO. Step D12 being identical to step C12 of updating the dictionaries such as implemented on coding with reference to figure 1, this step will not be described at greater length. Subsequent to the aforementioned step D12, there is undertaken, in the course of a step D13 represented in figure 4, the selection of the following quantized residual block of the current image IQ to be decoded. Next the steps described hereinabove of decoding the following quantized residual block are again implemented. If on completion of the aforementioned step D11, the updating criterion is not fulfilled, there is undertaken, in the course of the aforementioned step D13, the selection of the following quantized residual block of the current image IQ to be decoded. Next the steps described hereinabove of decoding the following quantized residual block are again implemented. In the course of a step D14 represented in figure 4, the decoder DO of figure 3 tests whether the current block which has been decoded in accordance with the decoding method described hereinabove is the last block of the current image IQ to be decoded. If such is not the case, the aforementioned step D13 is implemented. If the current block is the last block of the current image IQto be decoded, in the course of a step D15 represented in figure 4, the decoder DO of figure 3 tests whether or not the following current image IQ+i to be decoded is an image of Intra type. In the case where the following current image IQ+i to be decoded is of Intra type, before undertaking the decoding of the blocks of this image in accordance with the decoding method of figure 4, step D12 of updating the dictionaries is implemented. In the example represented, the quantization vectors of the dictionary CBKopt are each initialized to a predetermined respective value. In the case where the following current image IQ+i to be decoded is not of Intra type, there is directly undertaken the decoding of the blocks of this image in accordance with the decoding method of figure 4. The decoding steps which have just been described hereinabove are implemented for all the blocks Bi, B2, ..., Bu Bs to be decoded of the current image IQ considered, in a predetermined order which is for example the lexicographic order. It goes without saying that the embodiments which have been described hereinabove have been given purely by way of wholly non-limiting indication, and that numerous modifications can be easily made by the person skilled in the art without however departing from the scope of the invention. CLAIMS 1. A method for coding at least one image (IQ) split into blocks, implementing, for a current block (Bu) to be coded of said image: - a prediction (C3) of the current block in accordance with a mode of prediction selected from among a plurality of predetermined modes of prediction, - an obtaining of a predictor block (BPopt) on completion of said prediction, - a calculation (C4) of a first set of data representative of a difference between the predictor block obtained and the current block, - a comparison (C5) of said first calculated set of data with a plurality of quantization vectors, - a selection (C6) of one of the quantization vectors according to a predetermined coding performance criterion, - a coding (C7) of an index associated with said selected quantization vector, said method being characterized in that it comprises: - a calculation (C8) of a second set of data representative of a difference between the first calculated set of data and the selected quantization vector, - a coding (C9) of the second calculated set of data in the course of which at least one of said quantization vectors is modified (C12) as a function of the data of the second calculated set of data. 2. The coding method as claimed in claim 1, in which the modification of one of said quantization vectors is implemented only if the data of the second calculated set of data fulfill a predetermined criterion. 3. The coding method as claimed in claim 1 or 2, in which the modification of one of said quantization vectors is implemented with the aid of a parameter whose value depends on the size of the image to be coded. 4. The coding method as claimed in any one of claims 1 to 3, in which if said current image is of Intra type, the quantization vectors are each initialized to predetermined values. 5. A coding device (CO) for coding at least one image (IQ) split into blocks, comprising, for a current block (Bu) to be coded of said image: - prediction means (PRED_CO) for predicting the current block in accordance with a mode of prediction selected from among a plurality of predetermined modes of prediction (MPo, MPi,..., MPV,..., MPQ), delivering a predictor block (BPopt), - calculation means (CAL1_CO) for calculating a first set of data representative of a difference between the predictor block obtained and the current block, - comparison means (CAL2_CO) for comparing said first calculated set of data with a plurality of quantization vectors, - selection means (CAL3_CO) for selecting one of the quantization vectors according to a predetermined coding performance criterion, - first coding means (CB_CO) for coding an index associated with said selected quantization vector, said coding device being characterized in that: - said calculation means are activated to calculate a second set of data representative of a difference between the first calculated set of data and the selected quantization vector, and in that it comprises second coding means (MC_CO) which are activated to code the second calculated set of data, at least one of said quantization vectors being modified as a function of the data of the second calculated set of data. 6. A computer program comprising program code instructions for the execution of the steps of the coding method as claimed in any one of claims 1 to 4, when said program is executed on a computer. 7. A recording medium readable by a computer on which is recorded a computer program comprising program code instructions for the execution of the steps of the coding method as claimed in any one of claims 1 to 4, when said program is executed by a computer. 8. A method for decoding a data signal (F) representative of at least one image (IQ) split into blocks, implementing, for a current block (Bu) to be decoded: - a decoding (D3, D5): • of an index (IVopt) associated with a quantization vector (Vopt) which belongs to a set of quantization vectors, • and of prediction data in respect of the current block to be decoded, - a determination (D6) of said quantization vector associated with said decoded index, - an obtaining (D4) of a predictor block (BP0Pt) on the basis of the decoded prediction data, said method being characterized in that it comprises: - a determination (D1), in said data signal, of data relating to the current block to be decoded, - a decoding (D8) of the data relating to the current block to be decoded, in the course of which at least one of said quantization vectors is modified (D12) as a function of the decoded data, - a reconstruction (D9) of the current block on the basis of the decoded data, of said quantization vector and of the predictor block obtained. 9. The decoding method as claimed in claim 8, in which the modification of one of said quantization vectors is implemented only if the decoded data fulfill a predetermined criterion. 10. The decoding method as claimed in claim 8 or 9, in which the modification of one of said quantization vectors is implemented with the aid of a parameter whose value depends on the size of the image to be decoded. 11. The decoding method as claimed in any one of claims 8 to 10, in which if said current image is of Intra type, the quantization vectors are each initialized to predetermined values. 12. A decoding device for decoding a data signal (F) representative of at least one image (ICj) split into blocks, comprising, for a current block (Bu) to be decoded: - first decoding means (DB_DO) for decoding: • an index associated with a quantization vector which belongs to a set of quantization vectors, and prediction data in respect of the current block to be decoded, - calculation means (CAL1_DO) for determining said quantization vector associated with said decoded index, - prediction means (PRED_1_DO) for obtaining a predictor block (BPopt) on the basis of the decoded prediction data, said decoding device being characterized in that it comprises: - second decoding means (MD_DO) which are activated to decode data determined in the data signal and relating to the current block to be decoded, at least one of said quantization vectors being modified as a function of the decoded data, - reconstruction means (CAL3_DO) for reconstructing the current block on the basis of the decoded data, of said quantization vector determined and of the predictor block obtained. 13. A computer program comprising program code instructions for the execution of the steps of the decoding method as claimed in any one of claims 8 to 11, when said program is executed on a computer. 14. A recording medium readable by a computer on which is recorded a computer program comprising program code instructions for the execution of the steps of the decoding method as claimed in any one of claims 8 to 11, when said program is executed by a computer.