You might have read in a previous AdSense blog post that opting in to both text and image ads increases the pool of advertisers bidding for your site inventory — which can in turn increase the amount of revenue you make from AdSense. In the past, changing all your ad units to accept both text and image ads might have taken a lot of time. But now, the new interface allows you to easily find and edit all or some of your ad units with just a few clicks.
We recommend opting your ad units into both text and image to increase auction pressure, ensuring you will get the highest RPM possible. Keep in mind that some units such as link units and certain ad sizes cannot contain image ads, so you will not be able to modify these ad types.