How to get artist pictures in the music library?

Hello,

With the iOS 7 update, artist pictures in the music library seemed to be arranged by the iTunes store, it wasn't the album art anymore. When I first updated, I didn't have any artist pictures for about 2 hours, and then without any signal it updated I guess. Unfortunately my unlock button didn't work anymore, so last saturday I got a new iPhone 5 from the Apple Store. It was still iOS 6, so I had to update it to 7 and restore my iCloud copy. This didn't include any music, only contact info, apps etc. When I got home, I deleted my whole iTunes library so I could organize my music a little bit more. I added all my files, and synchronised. There were about 50 artists, and only about 4 had an artist picture. Even though EVERY album in my library has album art, no exceptions. I've tried deleting and resynching multiple times, but it didn't help. Though when I add like 1 new artist, it does give it an artist picture. Does anyone have an idea how to give the artists a picture? I also tried connecting to the iTunes music store, in case it might upload the pictures from there, but that didn't help.

note: all the albums do have album art

Example music library:

Example artist with album art but no artist picture:

Thanks in advance