Spammers use GIF animation to bypass content filtering
Kaspersky Labs documents the usage of GIF animation in spam messages that are designed to bypass content filtering controls.
See August 30th entry - "An animated August"
Example of "Hot Stocks" SPAM using GIF animation
QUOTE: We've recently detected yet another new trick being used by spammers. Spam now isn’t just being sent as a static graphical image in an attachment, but as an animated image. Spammers are using GIF animation which will be recognized and displayed by all popular browsers. Spammers are always developing new technologies in order to evade spam filters. Whether or not animation will make spam more difficult to detect isn't yet clear. It's true that a lot of spam filters don't analyze the actual graphics in spam.