Some SPAM filters use OCR to detect text in pictures like this one. This type of SPAM detection is very costly: all these nifty text recognition algorithms are running on your resources and are wasting your (CPU-)time. Additionally, due to spammer's creativity boost during the last two years even anti-spam techniques like fingerprinting with hash checksums are rendered useless when there's no image like the other one. Spammer's software just modifies the original picture, does some blur, some smear, puts abstract geometric patterns into the image, thus generating a unique piece of art, and will outsmart the fingerprint.
I'm getting these images every day. When they arrived for the first time I thought it was a joke, then I accepted the creative idea to fool my filters, then I got annoyed, as always, because it's just SPAM.
That message kept stuck in my filter, anyway, because it didn't like the gateway (just some contaminated Windows box, connected to the Internet via DSL or cable), and the mail's message ID was strange because it used a domain that doesn't exist.