I had the same problem with cover art but my friend and iTunes guru, bflat taught me. Double click on the album in iTunes scroll down to "Get Info". Then click on "Artwork" and double click on the album art displayed and click on "Delete Artwork". After that, click on "Add Artwork" and add a previously downloaded JPEG of the proper album art and click "OK". A bit cumbersome but that should solve your problem of the album artwork.