For me, text ads performs the best. But it also depends on the type of site you have. In some sites, text ads will perform better, where in some, image etc ads will perform better.
Yeah it would depend a lot on the type of ads you get for either text or multimedia. If your website brings up a really good multimedia ad, you might get a high click through rate. On the other hand, your website could also bring up a very enticing text ad. So you'd really have to test both to see what works best for your site/niche.
It depends on which niche you are in so you just have to test it out yourself. Some advertisers are willing to allocate higher budget for their campaign but generally image ads usually pay less per click compared to text based ads.
Make channels to make test on what kind of ads might worth better for you. I have serious type of site so I prefer text ads only. On game related sites, image ads are better for them.