pity you can't block spam with .GIF attachments... I mean... how many people send .GIFs???
Why not?
Depending on your email program, you can create a filtering rule that does just that...
You'll always have a line containing filename="<something>.gif" If your rules support wildcards or regular expressions it's a piece of cake to block those. If not, you can search for both the filename=" and the .gif" seperately, but risk some false positives.