I use the following steps when deciding on text ad vs image ads with adwords: 1) Start with text ads with higher cpc to starting capturing CTR 2) Ask merchant to place tracking pixel on thank you page. 3) Once you start converting, run placement report to see which sites are converting. 4) Create a new adgroup with image ads only for those sites that are converting. Image ads cost much more and you need to pay to cover all the other text ads that would have otherwise taken the placement. These are simplified steps, would be good to find out how others are using it.
Wow... nice tips. Hope I'll get more CTR by this method. I am going to try this method of ad placement.