Before MediaMonkey I used MusicMatch and I added to nearly every Song a Album Art.
Now, MediaMonkey supports Album Arts too, I can use this old Album Arts in the mp3s again.
When I look up in Files to Edit - Unknown Album Art, nearly every Song is listed - but some of them have a Album Art, i know.
When I now click on Files to Edit - Unsynchronized Tags, he now should recognize that some mp3s have Album Arts which MediaMonkey doesn't know about yet, or not?.
I solved the "Problem" to uncheck the "Only for files with changed timestamp or size" in the Library-options and to rescan the Library.
Is it a bug or a feature?