[exim] Banning a Specific Image

Pardon if the answer to this is too obvious, but is there a way to bounce an
email that contains a specific image. A group of Spam messages have been
getting through Spamassassin as well as SURBL and the one thing they have in
common is they include a graphic called "unknown.gif" - or at least that's
how it is showing up in the email code.

Is there a way to bounce all email that contains "unknown.gif"?

Thanks!