netmeg - 3:45 pm on May 6, 2013 (gmt 0)
It seems to go back and forth for me. I think it's partly because my sites are mostly seasonal, and my best ads may well be seasonal (travel, vacation, that sort of thing) and the advertisers may not run them in the winter months. Right now I'm in a text only period, I'll probably gradually start testing them again this month.
As a general rule, across my sites the image ads get higher CTR but lower EPC. YMMV.