According to IronPort's statistics, worldwide spam levels in October 2005 were 31 billion messages per day. That figure has risen to 61 billion messages per day in 2006.

25 percent of this spam was image-based compared to 4.8 percent the year before. The average message size also increased from 8.9 kilobytes to 13 kilobytes. Global spam contributed to more than 819 terabytes of bandwidth per day during 2006.

IronPort squarely lay the blame on the falling "catch rates" of spam on signature-based antivirus solutions. IronPort marketing vice president Tom Gillis said rapidly changing randomization techniques, the basis of image spam, are counteracting signature-based tools.

Adam Biviano, Trend Micro premium services manager, said image-based spam is one technique that does circumvent "legacy" security software, adding that is why the majority of tools now incorporate more than one form of detection method.

"If you look at 25 percent of image-based spam being hard to pick up with a signature-based tool that still leaves 75 percent of spam out there for which there are already signatures," Biviano said.