One thing to mention about Gracenote, at least on my 2013 F30 with NBT.
I have some tracks on an USB stick which I've imported on the car's HDD. The tracks didn't have any cover photo on them, at least they didn't appear in Windows Media Player when I've played them on the PC, but in the car the cover is there. My Gracenote DB version is from 02/12 I think, didn't update it yet, but the track that automatically got its cover was older (50 cent - Get in my car).
So Gracenote doesn't work only for Audio CDs
