I understand that. How then does SA treat messages mainly made up of images?
Hmm, in the context of what, bayes?
SA treats messages all in more-or-less the same fashion. embedded image based spams are only going to wind up matching bayes if the headers or URIs are part of bayes's header learning. SA's bayes doesn't learn from general HTML tags.
Really your best tools against image spams are SURBL (for ones that link external websites), and DCC or razor (for ones with embedded images).
Also, the HTML percentage rules kick in here, but their scores are pretty low these days.
