Your electing to not show ads with no images in your admin setup.
Basically the ad block works by viewing permissions and the setting above. We limit the query for performance reasons to grab the amount of images you want PLUS ANOTHER 10 and then out of that amount of ads we elect to display the amount of ads the user has permission to view from what is selected. That is of course after we subtract the ads with no images if you have that feature set to not display them. Thus you have 2 ads with images viewable.