They do check every image on an initial submission, but only spot check after that. If you feel their reason was questionable, you could rename and resubmit a photo mixed with with a bunch of new stuff. I haven't tried that yet, but intend to at some point.
Based on my experience I suspect, but have no proof, that they are prone to check every first image in a submittal, then at random after that. I rename my images in sequential order for Alamy specifically so I can more easily identify them, so I've started making sure each first image is plain and simple and easy to pass.