The easiest but time consuming way is this.
In the dir /data/web you’'ll find all html encoded folder names.
- Copy the missing Artist folder form
/data/albumart/web
to/data/albumart/personal/artist
- Remove the empty json file from these folders
- Place your artist image as ‘Artists.jpg’ in the folder that you just copied
- Repeat above steps for the missing artist.
- When completed, got to Sources click on “Reset Album Art Cache”
The fun part, it might be possible you miss previously found Artists and you can repeat this again.
At this moment I had to create 904 folders where most are pretty known Artist and also known by musicbrainz. This is about a 20% miss.
Like:
Bob Marley, Bon Jovi, Coldplay, Elton John, to name a few.
I’ve reported this a couple of times to @Volumio, but so far I have seen no improvements.