well...yeah...the site matters, your placement, use, colours...everything matter. however, i feel, images are far less boring than simple texts. texts makes your site look more professional, and the ads are there, making the viewer feel, you can click them if you want to, but you don't have to... but, images do grab more attention...that's how i feel... idk...upto you.
it's easy to try without putting much income at stake. 1) make sure that image ads are not listed as an option in all of your current ad blocks. 2) turn on image ads at the account level. 3) enable image ads on just a few ad blocks, and give 'em a specific channel, so you can track the results.