Good article on image spam:
A year ago, fewer than five out of 100 e-mails were image spam, according to Doug Bowers of Symantec. Today, up to 40 percent are. Meanwhile, image spam is the reason spam traffic overall doubled in 2006, according to antispam company Borderware. It is expected to keep rising.
The conceit behind image spam is graceful in its simplicity: Computers can’t see.
Definitely look at the interactive graphics page.