I'm posting this for anyone who is as slow as me...
I've often wondered about the relative performance of image and text ads--my assumption was that text, on my text-oriented site, would do better, but I had no proof. And testing didn't seem to indicate any difference. So I allowed images.
But in poking around in the AdSense reporting, I see that under "Performance reports" you can look at "ad types"--and see how text and images ads are actually performing. And what I found is that image ads perform FAR worse than image ads. Fortunately they only consisted of about 10% of my recent ad impressions.
I don't know when this information became available, but it's useful!
I'm sure that the relative performance of text and image ads will vary wildly from niche to niche.
For a specific niche there may only be a small number of image ads available. These may yield a higher click through rate (which will, of course, depend on your site) - however the earnings per click will depend on what the advertiser concerned is bidding - if image advertisers in your niche area are bidding low then you may het a high CTR with a low earnings per click (which may still be better than a text ad) - but if you happen to have an image advetiser in your niche which is also paying a high price per click then you could do very well - until their advertising budget is exhausted of course.