It really does not make sense to use only image ads. You will soon find out that the majority (about 75%) of the ads are text ads.
I think it all depends on the "target audience". I have had better luck using image banners than text ads to bring me traffic. My thinking is image ads seem to work better on arcades... as these users are more graphic orientated. But yes, I agree, it is better to find the right balance of image and text ads and also to find the the placement that works the best.