Just stumbled upon this old article at the IronPort site, explaining image-based spam. I think we all know the basics. Spam filters scan for text, images don’t have text, yadda yadda. But…

"Actually, it’s a little more complicated than that. Some spam filters
grew clever enough to spot simple types of image spam. At that point,
spammers came up with a fiendishly clever trick. They learned how to
use a layer of text on top of a layer of a randomly generated
background for each message."

Other interesting tidbits about where it all came from, pump and dump stock scams, and more at:

http://www.ironport.com/company/pp_webhosting_news_11-29-2006.html

We’ve seen a lot of experienced web designers actually send HTML emails that are nothing but one giant image. That’s an easy way of getting your campaign mistaken as image-based spam. We’ve posted a few other very, very common mistakes that we see web designers and creatives making all the time with their email campaigns:

Common Email Design Mistakes