1. An image processing apparatus including at least a processor for executing a plurality of control program units, the apparatus comprising: a selection unit, executed by the processor, configured to select a first image from plural images stored in a memory; a first print control unit, executed by the processor, configured to cause a printing apparatus to print, on a sheet, a sheet image including the first image selected by the selecting unit, information for specifying a size of a print medium selected by a user, and an area designated for the user to add an added image; an acquiring unit, executed by the processor, configured to acquire the added image which has been added by the user in the area designated on the sheet printed by the first print control unit, and a second image including the information, from the sheet, which is read by a reading apparatus; a determining unit, executed by the processor, configured to determine a selected size of a print medium selected by the user, based on the information included in the second image acquired by the acquiring unit; a processing unit, executed by the processor, configured to process both the first image retrieved from the memory and the added image acquired by the acquiring unit according to the selected size of the print medium determined by the determining unit, so that the size of both the first image from the memory and a combining image corresponding to the area designated on the sheet, including the added image, are fitted to the selected size of the print medium; and a second print control unit, executed by the processor, configured to cause the printing apparatus to print a third image, which is generated by combining the first image retrieved from the memory and the added image, as processed by the processing unit, on the selected size of the print medium.