A thread at DigitalPoint forums named do images pay more? discusses whether graphic ads over text ads or more profitable for the publisher. Everyone seems to agree that graphic ads pay the highest cost per click to the publisher, when compared to the text ad equivalent. However, there is argument in the thread, if a graphic ad has a higher or lower click-trhough rate when compared to a text ad.
I have personally heard from some large 3rd party PPC management companies, that they almost always opt to have their clients show image ads on the AdSense content network, when possible. They seemed very confident, based on tracking data, that they had a higher CTR and conversion rate then text ads.
In the thread, some say they have seen their CTR and CPC go up when turning on the image ad option in AdSense. But some members in the thread argue that everyone has "ad blindness" towards banner and graphic ads.