1. A method for comparing images, comprising: receiving a first image and a second image; processing the first image and the second image using a program executable by a processor communicating with a computer, the processing of the first and the second image including scaling the first image and the second image to a common spatial resolution, wherein the processing further comprises filtering at least one of the first image and the second image, and, wherein one or both of the first image and the second image are bitonal, having black pels and white pels, and wherein the filtering comprises removing black pels or groups of black pels surrounded by white pels, and the black pels or groups of black pels exist in a square grid having the size 1 by 1 pels to 4 by 4 pels; selecting one or more pel regions of the scaled first image and of the scaled second image; subtracting the one or more selected pel regions of the scaled first image from the one or more selected pel regions of the scaled second image, respectively; and determining whether the first image and the second image depict substantially identical subjects based on the results of the subtraction.