If you have missing artwork for the artist and web-art is enabled you might want to try the following:
Volumio looks at “Last.FM” for artwork. Check the spelling you have of the artist with Last.FM, do they use the same spelling, dashes and comma’s? If not, change your artist-name to the name used on Last.FM, update your database and hey presto: artwork!
For example, I had ripped and tagged and artist called “Fisher Z”, I tagged it using a respected tagging utility. It turned out the correct spelling is
2nd example: “Grp All Star Big Band” I had and the spelling on Last.FM is “GRP All-Star Big Band”.
A lot of work to check but in my case I found a lot of artwork.