To test, I have uploaded one folder.jpg file to one album in my library (21 from Adele), and refreshed the library (I have never used album art before). I have Sonos controllers on several devices, and the album artwork shows on some, but not on all:
- Android Samsung phone : no trace from the album art
- Laptop : shows small thumbnails in albums scrolling, and next to each songs once album selected, but nothing in the larger view when playing (left & middle of screen)
-iPad : nothing when browsing albums, but once the album selected, it does show a thumbnail; once playing, it does not show on the big window.
I just put one single folder.jpg, 500x500 px, 40Kb in the same directory than the songs. I would imagine that Sonos would use that to resize and display the jpg everywhere?