https://discussions.apple.com/thread/264826
I had a similar problem when I converted my iTunes albums to my Aurender server. For example, iTunes does NOT STORE the album cover art within the song folder but "HIDES" it in another encrypted and hidden folder. This means the Aurender setup coming from iTunes requires you to run Doug Scripts to extract the album cover art in your transfer. Doug's Scripts only works on Apple computers. The Script find the cover art images (from iTunes), changes the file name in my case, Folder.jpg. It was a time consuming process BUT works.
http://dougscripts.com/itunes/scripts/ss.php?sp=savealbumartjpeg
I suggest you search for finding cover art in Gracenote and determine if the covers art can be converted. This is not an easy task but I suggest you search and see what you can find. Good luck.