There was a very good question proposed by a member over at Digitalpoint forum. He asked about which Adsense ads made more money, the text or image ads? Its one that I think has already been considered but not fully investigated enough yet, due to the fact that images ads are still a bit new. You also only have selected sizes for images ads such as: Leaderboard (728 x 90), Banner (468 x 60), Skyscraper (120x600), and Medium Rectangle (300 x 250). Additionally Image Ads come and go. Some in the thread mentioned that the more broad use of keywords you have on the page, the more likely a broad matched Image Ad will show up eventually.
I am thinking based on observations of my own that possible Image Ads do in fact bring better revenue to publishers of adsense. They cost more from what I can tell, they appear to a wider audience, they are more visual, attract select groups, and Google can ultimately target this ads at particular audiences and sites, giving them the ability to fine tune the ad network, resulting in higher CTR's and more money for adsense publishers. But thats just a theory.
Check out the thread over at Digitalpoint: Image Ads vs. Text Ads
