Google AdSense has an ad unit named "link units." The link unit ads look like links and if you click on them, they take you a landing page of Google ads. In the past, those ads were only text ads. Now, Google is showing image ads on the bottom of the page.
Here is a screen capture:
Google explained more about how and when they are triggered:
These image ads are contextually targeted, based on the link unit topic users clicked to reach the ads. If no image ads are available for the given topic, we'll only show the original text ads -- in other words, we won't show a PSA or insert a blank space in place of the image ad in this case. Advertisers can bid on these image ads on a cost-per-click (CPC) and cost-per-impression (CPM) basis, so you'll generate earnings for valid clicks and impressions. You can review the performance of image ads on your link unit pages by generating a link unit report on your Advanced Reports page.
Here is a sample of a real link unit Google ad:
Forum discussion at WebmasterWorld.