I have a three-disc album where each disc is in a separate jewel case with its own cover art. The album already existed in MusicBrainz but lacked cover art. So I added it (3 front covers with a comment CD1, CD2, CD3). I note from a prior thread  and ticket  that cover-art per disc (medium) isn’t supported.
I have successfully tagged the three disc set of tracks using Picard but I cannot work out how to choose which cover art to apply to specific tracks. I tried selecting by clicking on the image displayed at the bottom right of the screen - I could see all the cover art there but I could not work out how to apply a spcific image to a selection of tracks.
Is it possible to do this ?
 Add cover art for each medium