Internetworkers: http://www.ibiblio.org/internetworkers/

[Geoff Davis <geoff AT geoffdavis.net>]

If you are using OS X, there are 3rd party spam filter plug-ins that  
work great on image spam.  SpamSieve ($30 at http://c-command.com/ 
spamsieve/ ) is awesome.  Maybe one spam message gets through every  
day or two (I get 100-200 spams a day).  IIRC, I trained it for a  
week or so; since then I've only had 2 or 3 total false positives  
since I started using it a couple of months ago.