DESCRIPTION OPTICAL DISC AND METHOD FOR IMAGE FORMING THEREON TECHNICAL FIELD The present invention relates to an optical disc on which an image can be formed by laser light and a method of forming an image on optical discs. BACKGROUND ART As optical discs such as a write-once digital versatile disc (DVD-R), etc., optical discs having an ink receiving layer (printing layer) on their labeling side (a side opposite to the side on which laser light is irradiated during recording or reproduction of optical information) are in use in practice. Users can print pictures or drawings on the printing layer using an ink jet printer, etc. Procedures for printing an image using an ink jet printer, etc. are advancing day by day, and image quality obtained has become exceptional. On the other hand, however, providing a new ink jet printer, etc., for forming an image is costly Furthermore, transfering an optical disc to an ink jet printer, etc. to form an image thereon, after recording information on the optical discs, takes time and effort. The task seems to be more complicated where information is recorded on plural optical discs and images are formed therefrom. Furthermore, when a printing layer is provided on the surface of the optical disc, there is a fear that problems of storability, such as deterioration due to moisture in the air, stains due to adhesion of dusts, etc., may arrise. Image forming apparatuses and methods for forming an image that can record an image having a high contrast ratio on an optical disc as well as recording information on the recordable side have been suggested (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2004-005848). Furthermore, several optical recording media on which an image can be formed by irradiation of laser light are known (for example, see IP-A Nos. 2000-113516, 2001-283464 and 2000-173096). These are convenient in that they can form good visible images. However, it is impossible to recognize on which side of an optical disc an image can be formed. Furthermore, quality control, etc. may become difficult since these optical discs do not comprise supplemental information, such as image forming information, etc. Therefore, there is a need for an optical disc in which a side on which image forming is carried out and a side on which image forming is not carried out can be easily distinguished and quality control thereof is easy. Furthermore, there is a need for an image forming method, which is suitable for image forming on such an optical disc. DISCLOSURE OF INVENTION According to an aspect of the invention, there is provided an optical disc on which a visible image can be formed by irradiation of laser light, which includes: a logo area, on which visible information is indicated; a pre-pit signal area; and an image forming area, on which a visible image can be formed by irradiation of laser light. According to another aspect of the invention, there is provided an image forming method of forming a visible image by irradiation of laser light on an optical disc, the method including; detecting a pre-pit signal from a pre-pit signal area formed on the optical disc; and forming the visible image based on the result of the detection; wherein the optical disc is the optical disc according to an aspect of the invention. BRIEF DESCRIPTION OF DRAWINGS Figs. lA and IB are partial cross-sectional views showing examples of the constitutions of the layers for the optical disc of the present invention. Figs. 2A, 2B and 2C are top views showing examples of optical discs of the invention. Figs. 3A, 3B and 3C are partial cross-sectional diagrams showing examples of the optical discs of the invention. Fig. 4 is a block diagram showing the structure of an example of an optical disc recording apparatus that can use the optical disc of the invention. Fig. 5 is a diagram showing the structure of the optical pickup which is a component of the optical disc recording apparatus. Fig. 6 is a diagram which explains the content of the image data used for the formation of a visible image on the image-recording layer of the optical disc by the optical disc recording apparatus. Figs. 7A and 7B are diagrams explaining the laser irradiation control for expressing the gradation of the image employed upon formation of a visible image on the image-recording layer of the optical disc of the invention by the optical disc recording apparatus Figs. 8A and 8B are diagrams for explaining the laser beam control method employed upon formation of a visible image on the image-recording layer of the optical disc by the optical disc recording apparatus. Fig. 9 is a diagram for explaining the laser power control performed by a laser power controlling circuit that is a component of the optical disc recording apparatus. Fig. 10 is a diagram showing returning light of the laser beam originally emitted by the optical pickup of the optical disc recording apparatus to the image-recording layer of the optical disc. Fig. 11 is a diagram showing an FG pulse generated by a frequency generator 21 which is a component of the optical disc recording apparatus based on the revolutions of a spindle motor, and a clock signal generated based on the FG pulse. Fig. 12 is a flowchart for explaining the operation of the optical disc recording apparatus. Fig. 13 is a flowchart for explaining the operation of the optical disc recording apparatus. Fig. 14 is a diagram showing a disc ID recorded on the image-recording layer side of the optical disc. Fig. 15 is a diagram showing shapes of the returning laser beams that are received by the light-receiving element of the optical pickup of the optical disc recording apparatus. Figs. 16A and 16B are diagrams for explaining the size of the beam spot of a laser beam that the optical pickup of the optical disc recording apparatus emits to the image-recording layer of the optical disc. Fig. 17 is a diagram for explaining a method for detecting that the laser irradiation position of the optical disc recording apparatus has passed the reference position on the optical disc. Fig. 18 is a diagram for explaining a method for detecting that the laser irradiation position of the optical disc recording apparatus has passed the reference position on the optical disc. Fig. 19 is a timing chart for explaining the operation of the optical disc recording apparatus upon irradiation of the image-recording layer of the optical disc with a laser beam to form a visible image. Fig. 20 is a diagram showing the image-recording layer of the optical disc at irradiation with a laser beam emitted by the optical disc recording apparatus. Figs. 21A, 21B, 21C and 21D are top views showing examples of the optical discs of the invention. BEST MODE FOR CARRYING OUT THE INVENTION OPTICAL DISC The optical disc of the present invention is an optical disc on which a visible image can be formed by irradiation of laser light, which comprises a logo area on which visible information is indicated, a pre-pit signal area, and an image forming area on which a visible image can be formed by irradiation of laser light. By the visible information indicated on the logo area, a side on which image forming is carried out can be readily distinguished from a side on which image forming is not carried out. Furthermore, quality control becomes easy due to the pre-pit signal generated from the pre-pit signal area. Furthermore, since the optical disc has an image forming area on which a visible image can be formed by irradiation of laser light, a visible image having high contrast can be formed efficiently As used herein, the visible information that is indicated on the logo area refers to an indication for distinguishing a side on which a visible image can be formed by laser light from a side on which an image cannot be formed. Examples of the indication include letters such as "DRAWABLE SIDE", "NON-DRAWABLE SIDE" and the like, and illustrations that enable ready discrimination of whether letters or images can be formed or not, and the indication is provided on the logo area so that, for example, the indication can be recognized from either of the drawable side or the information recording side (recordable side, which means a side on which information can be recorded). For example, for the logo area 702a of Fig. 2B, an indication "LABEL SIDE - DRAWABLE" is provided on the label side, and for the logo area 702b of Fig. 2C, an indication "DATA SIDE - NOT DRAWABLE" is provided on the recordable side. Furthermore, as shown in Figs. 21 A, 21B, 21C and 2ID, embodiments in which an indication "DRAWABLE SIDE" is provided on the drawable side of the optical disc (Fig. 21A) and an indication "NON-DRAWABLE SIDE" is provided on the recordable side (rear side) (Fig. 2 IB) are also possible. Alternatively, embodiments in which an indication "DRAWABLE SIDE" is provided on the drawable side of an optical disc and no indication is provided on the recordable side (rear side) (Fig. 2ID) are also possible. By using such indications showing that a side is drawable or that drawing is prohibited, the insertion of an optical disc into a drive, mistaking a side on which recording is prohibited for a recordable side can be avoided when image recording. Namely, a side on which an image forming can be carried out can be readily distinguished from a side on which image forming cannot be carried out. It is preferable to provide the logo area on the outermost surface of the label side, because printing design can be readily changed while using the same specification of the disc main body. Examples of the methods for providing such visible information include a method including providing a profiled surface corresponding to the indication on the substrate, printing methods such as screen printing, etc. The profiled surface can be provided on the substrate by providing in advance a corresponding profile on the stamper for forming a substrate, and forming the substrate using the stamper by injection molding. The pre-pit signal generated from the pre-pit signal area includes at least any one of the following information for image forming, i.e., the outer diameter (120 mm, 80 mm) of the optical disc; recording format of the image-recording layer (write-once, rewritable); intended purpose (multipurpose, specific purpose); configuration of the surface of the substrate at the side having the image-recording layer (presence or absence of a guide groove, having a smooth surface or a rough surface); information about the manufacturer of the disc; verification information, etc. In the optical disc of the invention, the image forming area may be positioned at a deeper level the logo area. Namely, the image forming area may be placed inside of the optical disc. In this case, the image forming area is not exposed to the atmosphere, dust, etc, and therefore, storability of the image forming area is excellent. In order to further improve storability, it is preferable to provide an image-recording layer as the image forming area, between the two facing substrates. When an image-recording layer is provided between the substrates, laser beam spots can be narrowed, whereby visibility can be increased and the time period for drawing can be decreased, as compared to recording an image on the surface. Therefore, storability of the image forming area can be improved and the problem of working distance for slim drives can be avoided. Furthermore, a more expensive look can be provided to the optical disc as compared to recording on the surface. The thickness of each of the facing substrates is preferably 0.3 mm to 0.9 mm. By adjusting the thickness in this range, a space corresponding to the thickness of the substrate is formed between the laser pickup and the image-recording layer, whereby a substantial large working distance can be secured. Therefore, even when a slim-type drive having a small overall thickness is used, it is not necessary to separate the position of an objective lens from the outermost surface of the optical disc by a long distance, which gives a remarkable design advantage for minimizing total thickness of the drive. Furthermore, approximately the same working distance can be readily secured in both cases when an image is recorded on the information-recording layer on the rear side and when an image is recorded on the image-recording layer on the obverse side. The constitution of the optical disc of the invention may be any of read-only type, write-once type, rewritable type, etc. Among these, write-once type is preferable. The recording format is not specifically limited and examples thereof may include concave and convex pit type, phase transition type, optical magnetic type, dye type, etc. Among these, dye type is preferable. Furthermore, examples of the constitution of the optical disc of the invention include the followings. (1) The first layer constitution, which comprises a first substrate, and an information-recording layer and a reflection layer formed on the first substrate in this order; and a second substrate, and an image-recording layer and a reflection layer formed on the second substrate in this order, wherein the reflection layers are adhered via an adhesive layer. (2) The second layer constitution, which comprises a first substrate, and an information-recording layer, a reflection layer and a protective layer formed on the first substrate in this order; and a second substrate, and an image-recording layer and a reflection layer formed on the second substrate in this order, wherein the protective layer formed on the first substrate and the reflection layer formed on the second substrate are adhered via an adhesive layer. (3) The third layer constitution, which comprises a first substrate, and an information-recording layer, a reflection layer and a protective layer formed on the first substrate in this order, and a second substrate, and an image-recording layer, a reflection layer and a protective layer formed on the second substrate in this order, wherein the protective layers are adhered via an adhesive layer. (4) The fourth layer constitution, which comprises a first substrate, and an information-recording layer and a reflection layer formed on the first substrate in this order, and a second substrate, and an image-recording layer, a reflection layer and a protective layer formed on the second substrate in this order, wherein the reflection layer formed on the first substrate and the protective layer formed on the second substrate are adhered via an adhesive layer. In the layer constitution examples (1) to (4), a pre-pit signal area is provided on the second substrate at the side having the image-recording layer, and a logo area is provided on the second substrate at the side opposite to the side having the image-recording layer. The above-mentioned examples of the layer constitutions (1) to (4) are provided merely for the purpose of explanation, and an optical enhance layer, a protective layer, etc. can be provided where necessary. Each of the layers may be constituted by a single layer or plural layers. The optical disc of the invention may be one having an outer diameter of 120 mm or one having an outer diameter of 80 mm. Furthermore, the optical disc may be of card type or may have various modified shapes. The logo area may be provided on two positions, i.e., inside circumference and outside circumference, and may have any shape other than ring type. In case where the optical disc has an outer diameter of 80 mm, the outer logo area is not always necessary. Fig. 1A is a partial cross-sectional view that shows an example of the layer constitution of optical disc 500 of the invention. The optical disc 500 includes a first laminate 520 including a first substrate 512, and an information-recording layer 514 and a first reflection layer 516 formed on the first substrate 512 in this order; and a second laminate 528 including a second substrate 522, and an image-recording layer 524 on which a visible image is recorded by irradiation of laser light and second reflection layer 526 formed on the second substrate in this order. The first laminate 520 and the second laminate 528 are adhered via an adhesive layer 530 so that the first reflection layer 516 and the second reflection layer 526 faces each other. A pre-pit signal area 600 on which pre-pits have been formed is provided on the face of the second substrate on which the image-recording layer is formed. Furthermore, logo areas 610A and 61 OB are provided in a concentric fashion on the face opposite to the face on which the image-recording layer is formed of the second substrate 522 by a printing method such as screen printing, offset printing, etc. As shown in Fig. lA, the logo area may be provided on the surface of the optical disc 500 by printing, etc. Alternatively, as shown in Fig. IB, a logo area 620 that indicates visible information may be formed by providing a indented portion (a concavity and convexity pattern) on the second substrate and forming a reflection layer thereon. According to such embodiment, so-called a pit art can be formed, whereby design property can be improved. In this case, the reflection layer can be formed by extending the reflection layer hned on the image-recording layer, whereby steps for forming the layers can be decreased. The reference characters in Fig. IB that are same as those of Fig. 1A have the same functions. Therefore, explanations of such reference characters are abbreviated. As shown in the examples of Figs. 1A and IB, so long as the optical disc of the invention has a logo area, a pre-pit signal area and an image forming area, the relationship of the positions of these areas is not specifically limited. In view of the storability of the image forming area, it is preferable that the image forming area is positioned at a deeper level than the logo area (i.e., the image forming area is positioned at a layer that is further from the outer surface than the logo area, for example, the image forming area is positioned at an inner layer than a layer having the logo area) Furthermore, it is preferable that the pre-pit signal area is positioned at a deeper level than the logo area. In view of the visibility of the logo area and the formed image, it is preferable that the optical disc includes the logo area, the pre-pit signal area and the image forming area provided in this order from the inside circumference of the optical disc. When the logo area, the pre-pit signal area and the image forming area are provided in this order from the inside circumference of the optical disc, and the pre-pit signal area is positioned at a deeper level than the logo area, it is preferable that the outer circumference of the logo area is positioned closer to the outer circumference of the optical disc than the inner circumference of the pre-pit signal area as shown, for example, in Figs. 1 A, 3 A, 3B and 3C. By this arrangement, it is possible to attain excellent appearance and also it is possible to ease required positioning accuracy thereby improving suitability for inexpensive mass production and yield. The optical disc may include, at an outer circumferential area, a printing area (which may be a logo area). In this case, the optical disc may include a logo area, a pre-pit signal area, an image forming area and another logo area, in this order from the inside circumference of the optical disc, as shown in Fig. lA. Specifically, the constitutions as shown in Figs. 2A, 2B and 2C, and 3A, 3B and 3C are preferable. Namely, as shown in Figs. 2A, 2B and 2C, it is preferable to form a logo area 702 on the label side of a substrate 720, and to form a pre-pit signal area 704 and an image recording area (image-recording layer) 706 toward the outer circumference from the pre-pit signal area 704. Furthermore, the partial sectional structure as shown in Figs. 3A, 3B and 3C are preferable. That is, it is preferable to form the image recording area 706 and the pre-pit signal area 704, in this order from the outer circumference, between the substrate 710 and the substrate 720. Moreover, it is preferable that the logo area 702 is provided on the upper surface of the substrate 720. According to such constitution, the innermost edge portion of the optical disc can be shielded and the visual effect to users can be improved. In Figs. 3A, 3B and 3C, the radius rO, which is an inner circumference end of the logo area 702, is preferably 8 mm to 21 mm, and the radius rl, which is an outer circumference end, is preferably 21 mm to 23 mm (wherein rOr2). The radius rl is preferably the same as radius r2 or larger (rl>r2), and more preferably, as shown in Figs 3A, 3B and 3C, the radius rl is larger than the radius r2 (rl>r2). The radius r4, which is an inside circumference end of the image recording area 706, is preferably in the range of 23 mm to 25 mm (wherein r3