1. A method of performing image processing on an image synthesized from a natural image and a computer graphic (CG) image, said method comprising the steps of: separating said synthesized image into a natural-image region and a CG-image region; computing an image-processing parameter for said image processing, based on said natural-image region; acquiring an intermediate image by performing said image processing on said synthesized image, based on said image-processing parameter; and acquiring a processed image by synthesizing said natural-image region contained in said intermediate image and said CG-image region contained in said synthesized image.