I've been using the java-randomized script for putting images above ads for awhile now. I noticed that the more images you put in the rotation, the lower the CTR there is. I figured it's probably because people keeps refreshing the page to see what new images there are in the rotation, causing a lower CTR relatively speaking. The quality of the images in rotation also contribute to this lower CTR. Visitors are most likely too attracted to the images and totally forget about the ads below. Bottom line, don't make your images too interesting and don't put too many images in rotation. I think 20 ~ 30 images are sufficient. Just my little observation.
I use simple images and they seem to work well. However, I have never rotated them. I only use static and as long as you follow the TOS you can use this method effectively.
CTR does not matter in the slightest in this case. Its only absolute figures you should be looking at.
I think that acnice image should even contribute to CTR, as the person, looking at it and liking it, is supposed to click on the ad being intrigued by a quality picture.
I think as long as you don't try to mess with the codes or the appearance of the ads you should be safe.
No Doubt that the earnings are the most important of all, but CTR matters too, better use a tracking script for your pages, that counts the unique visitors, and then you can make your very own CTR (Call it oCTR - Optimized click through rate <US patent pending>) LOL
Try putting next to it and not above it, if you use 300x250 size. If if you use the leaderboard, try putting it on the bottom, who knows, It might work. But like what angelos said, they see a random image and they think it's a link, which might have an effect on your CTR.