At the moment there is no way to find a book that has no image automatically. You have to do it the old fashion way, but this can be quite easy with the cover view. Scale the images down until quite a lot of them fit on screen (bottom left slider while in cover view). Select the books missing covers and drag them to a new collection to make it easier to work with them. You can then set your cover view settings back to normal or change back to table view.