Which database is music app on ios 8.4 querying for artist pictures? I wonder, which database apple uses for artist pictures in the music app on iOS 8.4 and how missing pictures for certain artists can be added?

Pretty sure it's something internal, but then again, that would make things even more complicated to add missing artist pictures.
I think the data comes from CDDB, or at least it used to, back in the day. You can edit (only add?) album art in iTunes by dragging image files onto the album art display. I don't know of any way to do it with only iOS.