To get the album cover displayed, you just need that image in the folder with the flac files.
\The Cure\Disintegration\folder.jpg
or cover.jpg
I size mine to 1000px or less and also use the same image as the embedded art, but embedded art does not display in Volumio as far as I can tell.
The artist images work as others have explained above.
Unless I’m misunderstanding you, it looks like it doesn’t work.
I guess at this point I’ll just give up and when I have some time I’ll sort out the library. Will be a pain to have so many folders with one or two songs in each of them.