Having Control over Your Google AdSense Web Site
As a publisher, you have complete control over the advertisement that runs on your AdSense web site. You can choose to run only image ads, only text ads, or a combination of both. Google recommends that you choose a combination of both to maximize your earning potential, but the decision is yours. When making your ad decision, you also have the freedom to choose which type of ads that will be shown on your AdSense web site or even on a particular content page.
As of right now, you are not able to differentiate your image ad click rate from your text ad click rate. You can of course, differentiate your click through rate from one AdSense web site as opposed to another AdSense web site. For example, you run only image ads on one site and only text ads on another site. By setting up channels to track both sites, you can see which AdSense web site has the better click through rate. However you have to take into consideration that there would other factors that will affect your click through rate. These factors includes the content of you site, the placement of the ads in general, the format of your ads and more.
Please refer to the Google AdSense Support Site for specific instructions on how to enable or disable image ads.