AdSense has added to their line-up of image ad sizes by adding both the Square (250 X 250) and Large Rectangle (336 X 280). With the large rectangle being such a common ad unit size, I was always surprised it wasn’t in the current image ad size lineup.
To turn on image ads, there are two ways to do it. To turn on image ads for the entire account, in the “My Account” tab, click the edit link for the “Ad Type Preference”. Then select the “Display text and image ads in all ad units”.
To only allow image ads on specific pages only, you will want to check “Image ads only” or “Text and image ads”.
Image ads are also available in these sizes (note this page still needs to be updated with the new sizes):
medium rectangle (300×250)
wide skyscraper (160×600)
It may take a while to see many image ads in the new sizes as advertisers build up ad inventory, so you may want to hold off on the “image ads only” selection for the two new image ad additions.