1. An image processing system comprising: area setting means for setting up a masked area on a base original sheet and a trimmed area on an extracted original sheet, positions of the masked area and the trimmed area being correlated with each other such that centers of rectangles circumscribing the masked area and the trimmed area are in alignment with each other; image input means for performing a reading operation on the base original sheet and a reading operation on a portion of the extracted original sheet, the portion including the trimmed area set by said area setting means; image processing means for performing a masking operation on image data of the base original sheet and performing an enchasing operation on image data of the portion of the extracted original sheet on the basis of a correlation of the positions of the masked area and the trimmed area set by said area setting means; and image output means, responsive to image data obtained by the masking operation and image data obtained by the enchasing operation, for forming a synthesized image in which an image of the trimmed area is enchased in the masked area of the base original sheet.