Not sure about the Q30 but I've seen this happen on the QX60.
Here's a few suggestions you can try and see if your album art will appear.
-set music source to USB on the stereo
-check to see if you have "show album art" on in the info center menu
Or in the case of a QX60 owner, the cover art was hidden by the Status screen covering up the bottom half of the nav display (the bit that shows you mpg, climate, etc).