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In any case, the question is probably academic for most AdSense publishers, because so far there doesn't seem to be an overwhelming demand for image ads. I expect we'll see more if and when Google succeeds in marketing AdSense to large mainstream corporate advertisers and ad agencies.
I expect that text ads perform better, because they canbe better integrated into the content and aren't highlighted that much as advertisement as Image-Ads are. Regular image ads normally suffer from banner-blindness, too.
Due to the better matching I expect that Image-Ads on Google perform better than Image-Ads from any other regular advertising network.