For a text base website, a img ad may have good CTR if the ads. is relevant to your content. If is because when visitor keep reading your words content, suddenly saw a picture, he/she may think it is part of your site and willing to click it to have further info. etc. (for logic thinking, people will bore to read long words content and try to see any picture to have better and faster understanding) For picture base website, text ad may bring better CTR. Because people will try click those text ads. and try to get more picture which relevant to your site. What you thing?
I think anything that will draw peoples attention to it gives you a better likelihood of it being clicked.
With good contents, equally good Adsense ads would be served resulting in clicks. An interesting image would not guarantee ads people want to click unless they are desperate to get away (from a boring site).
adding images (especially relevant ones to the website) next to adsense ads can get you in trouble by google.. see : http://forums.digitalpoint.com/showthread.php?t=157565&page=2&highlight=images+on+adsense