Your best bet is to a/b test image ads versus text ads with the same size ad unit and with the same placement. the data from that test will give you sufficient information that will help you determine the use of image ads vs. text ads. i presently use image ads and i am seeing a better ctr with them over text ads. based on my historical data, text ads have not performed as well as they use to for me. i think site visitors are becoming "ad blind" to adsense text ad units.