What file is Roon currently using ?
Do the “various jpegs” in the artworkfolder just show a common box set cover ?
If so then doing a batch change outside Roon to change the name of artworkfolder to something like “boxart” should see the cover.jpg used.
Roon will use the “best” cover it can find, which appears to be the box set cover. If you can hide that cover from Roon then it will use cover.jpg.