Re: how to block image based spam?
Exchange 2003 comes with two notable spam filtering methods 1) IMF 2) DNS blacklists. Blacklists are controvertible and many disagree with that method. I however am a big fan, as long as legit emails that get blocked receive a clear and concise message on how to correct the issue (meaning: remove themselves from the blacklist)
In your case, I am familiar (not by choice:evil:) with the spam you are referring to, and would recommend the blacklist.