iTunes unable to change Artwork for Albums
try change the image format in .png 'Important: iTunesArtwork icon images should be in png format, but name them without the .png extension.' source: https://developer.apple.com/library/content/qa/qa1686/_index.html

Edit: Ultimately ITunes used the cover of a pdf from iBooks as a generic image, to solve this problem I found this solution that works for me:
"Choose the 'Options' tab in Album Info. The media kind would be 'Mixed'. Change it to 'Music'. Now you can see a pdf as part of the album. If you delete the pdf, you can now set your album artwork."
source: https://discussions.apple.com/thread/7127569