From the New York Times:
To the antispam researchers at MessageLabs, an e-mail filtering company, each new wave of a recent stock-pumping spam seemed like a personal affront.
The spammers were trying to circumvent the world’s junk-mail filters by embedding their messages — whether peddling something called China Digital Media for $1.71 a share, or a “Hot Pick!” company called GroFeed for just 10 cents — into images.
In some ways, it was a desperate move. The images made the messages much bulkier than simple text messages, so the spammers were using more bandwidth to churn out fewer spams. But they also knew that, to filters scanning for telltale spam words in the text of e-mail messages, a picture of the words “Hot Stox!!” is significantly different from the words themselves.
So the bulk e-mailers behind this campaign seemed to calculate that they had a good chance of slipping their stock pitches past spam defenses to land in the in-boxes of prospective customers.
It worked, but only briefly.