Disclaimer: This is UNTESTED, and I had to make an educated guess about which file to edit since I don't have a FW dev setup on this machine right now.
I noticed that when expanding the play queue, the current album cover would be shown at about 450x450, yet only the 200x200 version of it would be fetched from the server. This of course causes ugly artifacts, hence let's use the higher-resolution image instead.