Thanks for the quick response! That didn't quite fix the problem, but it did give me an idea. With a bit more playing about, it seems that the panel that was being left blank was meant for a marquee image - this is why nothing was showing when it was a console game (except in a few cases where some kind of marquee must have been scraped). Setting this to show box art as second priority to marquees fixed the issue isn most cases.
After a bit more playing, it looks like under images - game image type, this was set to marquee. Setting this to boxes cleaned up any other image problems across the system. I think that this second option may have fixed everything without having to play around with the priorities, but everything is working now, so I'll leaving it as is.
Thanks again.