Image spam has become the "delivery of choice" by new-school spammers. Why? It's difficult to detect and can bypass the older-school forms of spam filtering and in the ongoing competition for viewing, he who's spam is most viewed wins.
This great article explains the most common forms of image spam, how it's designed down to the finest detail to prevent detection and how some OCR software used to detect image spam is color-blind and the spammers have determined how to use this simple limitation to their advantage.
