Perhaps this will help:
Cover art does not show up on my Lexus either. But I'm not concerned -- I'm glad to get the music via Bluetooth. And I see the artist and song title, so I'm satisfied.
I believe that the Sync system will show cover art from media files from iTunes - but only when in the USB mode. I think the extra bandwidth from USB allows the artwork capability. There just isn't enough capability to move music, song information and cover art in the Bluetooth mode. I push my iTuens media files to USB and plug it in to get the full package. For more info, set the website FordTechLane