Normally, anti spam tools work by scanning text messages. In order to fool such tools, spammers shift their strategy from using plain text to using embedded images to send their messages. It makes the spam messages become undetected by traditional spam filtering software.

The effects of this type of spammmy intrusion are worse than the older one. Images take more bandwidth than text. The increasing size and the sheer volume of image-based spams have forced many businesses and average people to block all emails that contained embedded or attached images.

The prevalence of this form of spam increased in 2006, primarily utilized for advertising penny stocks. By early 2007, image spam reached an all-time high. It was used to advertise everything, from sexual enhancement to fake pharmaceuticals.

Anti spam technology today can detect image spams. They utilize some techniques such as Optical Character Recognition. However, although spam filtering software or services have been able to reduce image-based spams, many spammers have also turned to another strategy. It is like the cat-and-mouse game.

As anti-spam technology catches up to new techniques, spammers also continue to innovate and change tactics to deliver their spam messages. Besides sending out PDF spams, they also use animated images, blurred text messages, and even vary the geometry of images.

Perhaps there are no best approaches facing this problem. However besides installing a good anti spam tool, you can try to not downloading images that come from untrusted sources. Some email client programs have already provided this feature.