HawkWings.net has a good write-up on MacInTouch reader Bill Benson’s rule for fighting image based spam. Basically, most of this image spam contains a heading Content-Type which contains multipart/related. You have to add the header to your rules in Mail.app, but that’s the only tough part. I would recommend you set the filter to change the background colour until you know everything is working.
Update: You may want to add something like “From” does not contain [email protected].