I've been meaning to test this out, but I have a theory based on many of Google's own ad properties, that one 728x90 locked on image ads, and one 300x250 text/image box might yield the best CTR.... I'm guessing the Image ad pays less, but may in fact be more often clicked and appeal to a different viewer then the text box. You know, the whole left brain, right brain thing.
Image Ads may lead a user NOT to click due to 'banner blindness' The fact that text ads LOOK like hyperlinks make users click (just like following a link from one page to another). But then again, this is just theory.
Just edit the CSS, and put your ad code wherever you want the ads to display I would have thought that images would have a higher CTRs, like, Id be more inclined to click a picture advertising a game than click some text describing it...