1. A method of comparing images, said method comprising the steps of: representing a first image as a set of blocks of pixels; matching each of said blocks from said first image to a second image; recording a substitution as comprising said errors of said matching said blocks; recording a deletion as comprising a first portion of said second image not involved in said matching said blocks; recording an insertion as comprising a second portion of said second image involved in matching more than one of said blocks; and determining an image edit distance between said first and second image comprising said substitution, said deletion and said insertion.