That's strange, I think that depends on one niche to another. Mine is always the reverse cause only text ads pay well.
I do not think that this work for everyone. In image ads you will be able to see same kind of images in most of websites which you surf. Hence you will get less and less interest to click on that. But if you will use text ads and placed on the mid of article than people will read that and click on that one too. For me image ads never worked.
I have been told that you have to test your sites and do it over and over again to see what goes wrong or right. And you need to test it over a period of time for example, aweek or so. Whether your site is doing better with image or text depends on these tests.
contents site=images ads are more appealing, images site=text ads are more appealing too many text=too dry too many images=too wet you need to balance it out one way or the way