I found that because I used Media Player to rip/name my music that the cover art was a small thumbnail.
Probably like most people, I had to go through every album and insert the cover art I wanted.
It took time, I just used to do 10 albums a day until it was done, but it meant that I had the correct cover art, of a quality that satisfied me.
Then I backed up all my music with the new cover art
