I've observed that form based banner ADs have lowest CTR and even though they are images/flash, they dont pay you much if you are publisher. because, most forms have multiple fields to be filled and only when your visitor fills in all fields correctly and clicks on submit button, is his/her click counted. else they would just occupy your webspace paying you nothing i recently placed one such site in my filters list and my CTR increased from next day with same number of impressions