AdSense have added image ads to the bottom of link unit pages, this is great thing to increase the revenue. Also google is promoting link unit adds.
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.
If you aren't using link units on your pages, we encourage you to give them a try. They don't take up much space and can help add to your AdSense revenue --