1. An image processing device comprising: an input image acquisition portion that acquires an input image; a past image acquisition portion that acquires a past image of a photographic subject in the input image; a mode selection portion that selects one of modes, using the input image, from among a plurality of modes including a first mode in which the photographic subject in the past image is overlapped with the photographic subject in the input image and a second mode in which the photographic subject in the past image is arranged side by side with the photographic subject in the input image; and a display control portion that superimposes the past image on the input image in accordance with the mode selected by the mode selection portion, wherein the mode selection portion selects one of the modes in response to a motion of the photographic subject in the input image.