Google started showing contextually targeted image ads, on the pages, users reach after clicking a link unit topic. If no image ads are available for the given topic, the original text ads are shown, instead of PSA or blank space. Advertisers can bid on these image ads on a CPC and cost-per-impression (CPM) basis. You can review the performance of image ads on your link unit pages by generating a link unit report on your Advanced Reports page. Google claims their testing showed revenue improvements for many publishers.