larger the ads size more you get exposure, so I always go for 368x280. you can check this on my blog also.
It depends on the site. For my adult related sights its all about the image. For my technical sites its more about blended text ads. As for the size its just about placement and what looks good.
My vote goes for text ads. I can tell when image ads are showing up on my blog, because my revenue drops substantially. I have since gone in and only allow text ads at this point.
Based on my experience, text ads have more clicks than in image ads. But it's also good to use image ads if you are running a gaming related site.
The problem is that you all THINK but you don't know! Image ADs are CPM Ads and you are payed for each impression. Text ADs are CPC based and you get payed per click. Every AD competes in an auction for your AD spot, the one that has the highest bid multiplied by the quality score and smart pricing wins the auction. If you see an image AD then the CPM for that ad is higher then what you would get on avarage for text ads CPC per thousand impressions. The more competition there is for one AD unit the higher your earnings. Sometimes it is good to read the inside adsense blog to get an idea about the business you are all involved in Here is a pointer: http://adsense.blogspot.com/search/label/Earnings/Reports