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The images fool some filters because they have no easy way of knowing whether a graphical file contains an innocent photograph of a friend's birthday party or embedded text pitching Viagra or a company's stock.

The development marks yet another escalation in the battle between spammers and filter developers: As software gets smarter at detecting junk, spammers get smarter at fooling the filters.

Until last year, the use of image spam has been in decline as anti-spam filters figured out how to detect it often by applying a mathematical formula to known spam images and generating a unique signature that software can use to flag junk, said Craig Sprosts, senior product manager for anti-spam vendor IronPort Systems Inc.

But earlier this year, tools began circulating among spammers to automatically vary images ever so slightly a change in color here, a slightly larger border there. That changes the signature, helping it escape detection.