Text ads will perform far better than image ads. Image ads suit better for suits that have low bounce rate and users are returning.
Can I conclude then that you have to test it out and go with the one that converts better? Has anyone done that? How long does it take? What's the cost involved?
You will get a higher CTR with text ads. And you can increse it by also eliminating the border around your ad (making it less prominent contrary to what most would think) and making the background of the ad the same color of your webpage. Joel Comm has a great eBook called "AdSense Secrets" that will increase your CTR 10 times fold if you follow his tips.