There are three settings you can choose in AdSense for what kind of image ads to display: Text Ads Only, Text and Image Ads or Image and Rich Media ads. Which one should you choose? Will one result in higher click throughs and conversions? Here are the pros and cons, as well as the approach you can take.
The Pros and Cons of Text Ads
There are a few big things text ads have going for them.
People are also less likely to have "banner blindness" towards text. If your ads are well positioned and have good fonts / colors, you have a very good chance of getting read and getting clicked.
They can also blend into your overall site layout a lot more, resulting in a better-looking site. You can make it look like an integrated part of the design, rather than a glaring advertisement.
The downside of text ads? They don't stand out. There's nothing that grabs attention about them. Once someone is used to the AdSense ads on your site, it can become very easy to just skip over them.
The Pros and Cons of Image Ads
Image and Rich Media ads have the main benefit of being able to draw a lot more attention. They can use animation, giant headlines, etc. to get your user to click on the ad.
However, it's important to remember that only one ad will be displayed. In other words, that one advertiser has to perform well enough to make up for showing one ad instead of 3-4 text ads.
Keep in mind that it's in Google's best interest to help you make more money. The more you earn, the more they get as commissions.
That said, the auction process could take a long time. So to answer the question ...
Should You Run Image Ads?
The answer is yes. But split test them against text ads. You can do this either using a split testing script like Google Website Optimizer, or just running each for a week or two and switching them out.
Figure out which one works better for your site. Then run with the winner going forward. This is definitely a test that should be done, because the difference can be quite dramatic.