1. An image processing apparatus comprising: a display unit which displays an image; a detection unit which detects a position on the display unit designated from an outside; an original image display unit which displays an original image based on original image data in a first display area on the display unit; a specifying unit which, when a position within the first display area is designated from the outside, specifies a designated region corresponding to the designated position in the first display area based on a detection result of the detection unit by adding a width to a locus of the designated position detected by the detection unit, as the designated region; a combining unit which displays at least a part of an additional image based on additional image data in the designated region in the first display area to display a combined image, in which the original image and the additional image are combined, on the display unit, and an additional image display unit which displays the additional image in a second display area adjacent to the first display area on the display unit, wherein a size of the designated region changes based on the designation of the position within the first display from the outside, wherein the specifying unit specifies a corresponding region in the second display area that corresponds to the designated region in the first display area, in response to the designated region in the first display area being specified, wherein the specifying unit updates the designated region according to a change of the designated position detected by the detection unit, wherein the combining unit combines a portion of the additional image, which is displayed in the corresponding region specified in the second display area, in the designated region of the original image to display the combined image, and wherein the combining unit updates display of the combined image based on the updated designated region as the specifying unit updates the designated region.