We have created a tool for finding your listings that have no photos attached. Please look in the "At a glance" menu for the line "Number of listings without photos". Click on the number in green to show a list of listings with no photos.
If you see books on that list that do have photos attached to them, it is most likely because you have performed that search during a re-indexing cycle, and it may take an hour or two for those books to disappear from that list.
Click here for Biblio's recommendations and minimum size requirements for book photos.