Google announced they will now be displaying image banner ads on the bottom of the link unit ad pages. A link unit is an AdSense ad that takes the user to a page of ads. Those ads have always been text based ads. Now Google will be showing an image ad that is contextually relevant to the content of the previous page.
Here is a picture:
Google explains that the image ad is contextually targeted. If there is not image ad inventory, Google won’t show an image ad. The image ads are served by AdWords advertisers on a cost-per-click (CPC) and cost-per-impression (CPM) basis.