More faulty logic
Eventually, perhaps, I'll learn to think things through properly, but for now it seem that trial and error just leads me to more errors and more trials. My improved logic for detecting whether a post was a Review or a Listen failed to detect that it was neither of those things. As a result, those pages were having an empty image thrust upon them whether they wanted one or not. Now fixed.
But now I really need to work out why pagination has stopped working entirely. Something about how Grav handles a collection based on a category, I'm sure, but what?