1. An image processor comprising: a partial image extracting unit that extracts first partial images from a first image, and extracts second partial images from a second image; a sequencing unit that determines an order of the extracted first partial images in accordance with positions of the first partial images in the first image, and determines an order of the extracted second partial images in accordance with positions of second partial images in the second image; and a difference extracting unit that compares each first partial image with the corresponding second partial image in accordance with the order of the first partial images and the order of the second partial images, and extracts a difference based on the comparison between the first image and the second image.