1. An image generation device for generating high-definition image data representing a high-definition image higher in definition than an images represented by a plurality of image data, the device comprising: a target image size setting module configured to acquire output device information relating to an output device used in output of the high-definition image, and set a target image size expressed with number of pixels of the high-definition image suited for outputting by the output device based on the output device information; and an image composing module configured to select a plurality of source image data arranged in a time sequence from the plurality of image data, and compose the source image data to generate high-definition image data representing a high-definition image with the target image size.