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.