Ads are shown on Leaderboards for the Free Account holders. When you upgrade to an Executive Member, you have the option to shut off System Ads (by un-checking the 'Show System Ads' on the 'Ads' page), and add your own League Ads. Any size ads will work, but you'll want to adjust the ad size to fit the ad space. Both text and image (.png, .gif, .jpg) ads work.