As you may recall, 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 the changes used to take a lot of time. But now, the new AdSense interface allows you to easily find and edit all or some of your ad units with just a few clicks.
Simply go to the 'My ads' tab in the new interface and under 'Content,' choose the 'Ad units' page. Using the checkboxes, select the ad units you'd like to edit, or simply select the checkbox at the top of the list to select all of your ad units.
Using the 'Actions' dropdown, select 'Edit ad type.' Choose 'Text & image/rich media ads' and then click 'apply ad type.' Now any ad units that can be opted into both text and image will have this set as their ad type.
Note: That some ad units such as link units and certain ad sizes cannot contain image ads, so you'll not be able to modify these ad types.