I also would head into that direction. There is a German thread that deals with a Philips TV
EIGENES Coverbild in MP3 Datei einbinden
It claims that the Philips support told to limit the covers to 600*600 pixel and 300 kb (it also mentions 49 kb but that seems to be the size of the covers in bought files ...)
Anyway: I would check the size of files.
As mp3tag does not show the dimensions, it could be a possibitly to ...
... export the cover files (e.g. to a file %album%.jpg) to the file system
... use the windows explorer to search the *.jpg files and sort it by dimension (you can add that column in the list).
Perhaps it is possible to find a rule for files that behave properly and those that don't.