Well, that was pretty straightforward! I quit Plex, and reattached the sleeve art to my files - no errors. Incidentally, I noticed that I had previously attached other versions of the sleeve art to some of my files, so I deleted all sleeve art from every file and re-attached only the sleeve art I want.
I recall someone recommending that I make all my sleeve art files 400x400pixels. Some of the troublesome ones had previously been more that 400x400. Could that be why mp3tag doesn't like them?
After I had started Plex again, I noticed (after I ran the Plex process, 'Scan Library Files') that Plex still wasn't recognising am few albums correctly. What has greater priority, Plex or mp3tag? Sorry if this is covered in the help section.