1. An image processing device that generates a photo-mosaic image, the image processing device comprising: one or more processors operable to: detect a subject in each of a plurality of material images and identify a type of the detected subject in each of the plurality of material images based on object recognition on the plurality of material images; detect a shape of each block of a plurality of blocks obtained by dividing a production target image; select one of the plurality of material images for each block of the plurality of blocks, each of the selected material images having the type of subject matching the shape of a corresponding block of the plurality of blocks; perform a process on each of the selected material images to conform each of the selected material images to the shape and size of the corresponding block such that a region of the subject of each of the selected material images is within the corresponding block; clip a subject-including region of each of the selected material images which has been subjected to the process and generate a plurality of clipped images in which the subject is included; and set one of the plurality of clipped images as a replacement image for the corresponding block of the production target image based on color-related characteristics of the corresponding block and replace the corresponding block by the replacement image to generate the photo-mosaic image.