I have noticed the tex ads pay higher than image ads now when I moved over to adbrite I noticed the same image ads that had showed on the site pays at least 3x higher than adsense or more. I still use adsense as well but strickly tex and adbrite is stictly manauly approved banner's.
Basically text is higher. It is because those high CPC ads all are text link. Most of the images available is low CPC niche related ads