From ThisBlueWiki
Give a little nudge in SpamAssassin to single GIF attachments, maybe 0.3. Would put some spams over the edge.
Spam image DNSBL: Flatten spam images, desaturate, find edges?, blur, and generate a pixel-flipping resistant score by looking at n x n cells of the image instead of individual pixels (or similar -- investigate image similarity algorithms for this). Turn that into a base64 string that can be put on a "hostname" and looked up over DNS.