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:

Here is a sample of a real link unit Google ad:

Forum discussion at WebmasterWorld.