Google AdSense now displays an image ad to the bottom of each of link unit pages, and they indicate that their testing showed revenue improvements for many affiliates.
Just like regular AdSense, these image ads are contextually targeted, based on the link unit topic users clicked to reach the ads.
In the event there are no image ads for the given topic, they’ll only show the original text ads (no PSA or blank space instead of an image ad).
More details at http://adsense.blogspot.com/2009/05/image-ads-join-link-units.html.