1. A method of performing image processing on an image synthesized from a natural image and a computer graphic (CG) image that has a single color, said method comprising: utilizing a computer processor to perform the steps of: receiving via a computer input device designation of a region containing a natural-image region in the synthesized image; temporarily dividing the designated region into the natural-image region and a CG-image region by extracting from the designated region pixels that have the same color as the color contained in a region of the synthesized image other than the designated region, wherein when a first small region composed of pixels having the same color as the color contained in the region other than the designated region is present in the designated region if the size of the first small region is greater than or equal to a threshold value, the first small region is extracted from the designated region to be classified as part of the CG-image region, and when a second small region composed of pixels having a color different from the color contained in the region other than the designated region is present in the designated region if the size of the second small region is greater than or equal to the threshold value, the second small region is not extracted from the designated region and is to be classified as part of the natural-image region; dividing said synthesized image into the natural-image region and the 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.