1. An image generation device for generating high-definition image data representing a high-definition image higher in definition than 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, the target image size setting module setting the target image size with reference to an output image size set 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, wherein the target image size setting module calculates a ratio of the output image size and a size of an image represented by the source image data as an output image size ratio, sets the target image size equivalent to the output image size if the output image size ratio is smaller than a predetermined threshold, and sets the target image size to a value found by dividing the output image size by an adjustment coefficient value larger than 1 if the output image size ratio is above the predetermined threshold.