I think it's a solved problem. If you've got 10M users and they all get the same message (or one that's slightly different), it's a good bet that it's spam. ;-) Bayesian filters can be trained to reject such things.
SpamAssassin has various [link|http://systems.cs.uoregon.edu/Solaris/spamassassin.php|rules] regarding images.
I'm reasonably sure this problem has been solved. But I'm no expert. If it's continuing to concern you, check the archives of the [link|http://wiki.apache.org/spamassassin/MailingLists|SpamAssassin mailing list] or ask there. They should know.
HTH. Luck!
Cheers,
Scott.