1. An image synthesizing apparatus for synthesizing a subject image with at least an additional image to produce a composite image, comprising: an image input device for inputting image data of said subject image into a memory; an image processing device for processing said image data to synthesize said subject image with at least an additional image; and a mode switching device for switching over said image synthesizing apparatus between an overlay mode and an inlay mode, wherein, in said overlay mode, said image data of said subject image is input into said memory in a size corresponding to a designated print size of said composite image, and at least an additional image is overlaid on a predetermined portion of said subject image, whereas, in said inlay mode, a mount area is defined in said memory in correspondence with a designated print size of said composite image, and said subject image and at least an additional image are inlaid in those ranges which are defined in variable sizes at appropriate locations within said mount area.