I think the reason people are having such mixed results may be that only the song metadata is streamed over bluetooth and the album art is not. In fact, it appears that the car either has an onboard database of album art or it performs a lookup into their own servers for that art.
I too stream Spotify and I would say that 85% of my album art shows up. But I've also noticed about 5% of the time I get different album art on the car screen than shows up in Spotify on my phone. And the rest of the time I get no album art at all. But I will say that it's consistent. If I see art for an album, it will always show art for that album.
I'm going to guess that they may have a database in the car to save bandwidth or at least do some kind of caching in the car. And this database may be seeded by the Radio provider (Slacker in the US and Spotify abroad).
If you are not seeing album art over Bluetooth, try another album. Especially try more popular albums. The more esoteric stuff I listen to is what seems to be missing.
Also, if you're streaming Spotify over bluetooth consider checking out TSpot:
T-Spot
It's a great front-end to touch control your phone from the Tesla PC. It's a little slow to respond, but I'm hoping with the upcoming browser upgrade that might be fixed.