I'm not sure i'm on the same page right now . What isn't working, is it the local database which only stores artist/album images or is it MPDs track database? if it is the MPD trackdatabase you just need to Select "Settings->Update database".
If it is the local metadatabase for images, album/artist art will download automatically if new songs are played or you do it manually via Settings->Database settings->download artist images and download album images.
Artist/AlbumYear-AlbumName