1. A computer-implemented method for detecting spam images within a stream of messages comprising: extracting email messages having images with specified characteristics from the stream and performing the following operations only for those email messages: generating a generic signature and an image-specific signature for a first images within the stream messages; determining if the generic signature of the first image matches a generic signature of a second image; determining if the first and second images with matching generic signatures have different image-specific signatures; and including the matching generic signatures of the first and second images and one or more additional images in a spam filtering engine if a number of images with matching generic signatures but different image-specific signatures including the first and second images reaches a specified threshold value.