Having read all the threads I could find on this topic on these forums, and having experimented with .jpeg, progressive .jpg and non-progressive .jpg files, importing directly into iTunes from the Discogs listing for a particular album, the cover art still won't show on the Sonos controller with certain albums.
Running Mac OS 10.14.2 on a 2018 MacMini, and using the latest iTunes and Sonos software versions, I am using XLD to rip to .aiff and have been able to add all album artwork into iTunes for most albums but there are a few that just will not show. Very odd.
No finger pointing, but this appears to be some sort of an incompatibility between iTunes and Sonos.
Is there a definitive solution to this problem?