I think I found a solution .......
In Preferences - Files the location of my media pointed to My Music from iTunes, that was on my disc. Music was linked to those files. I double checked if there were no other references to the iTunes folders and there were not.
I deleted allllll my albums from Music, without deleting the song-files/mp3's/etc.
Then I pushed the 'Defaults' (in Dutch...) button and the Media folder changed from the iTunes location to the location in the Music folder ....
Then I went tot the iTunes Media folder in Finder and dragged allllllll my media to Music. I copied all the media WITH (most of the) album artwork!
After the I backupped the former folder and deleted them .... Everything works WITH album art now.