buckworks - 4:36 am on Oct 31, 2010 (gmt 0)
I believe that ALL image ads are CPM
That is not accurate. Advertisers can bid either CPM or per click on image ads, and they'll compete normally in the auction with all available ads.
From the advertiser's side, at least in my experience, good banner ads get a significantly higher click-through rate than text ads with the same targeting.