1. An image processing apparatus comprising: means for storing digital data representing a first image and a second image, each including at least an image portion of a common area, common to the first image and the second image; means for storing digital data representing a mask pattern image for extracting only digital data representing a plurality of two-dimensionally discrete images from the first image; means for generating digital data representing a synthesized image comprising a plurality of two-dimensionally discrete images extracted from the first image by the mask pattern image and arranged corresponding to the first image; means for displaying the synthesized image and the second image in an overlapped state; and means for changing a positional relationship between the synthesized image and the second image displayed in the overlapped state by said means for displaying.