I did not know that.
I've been embedding something like a 500x500 or 600x600 in all my individul mp3's, so I get the cover art no matter where I play them. I was thinking if I had a poker gathering and wanted to display the cover art on my comp or TV, the higher the resolution, the better.
So far, everything i've played them on has displayed them properly. (Zune 80G, WMP vista, Insignia player, and few others). They even played in my old JVC car stereo before I sold the car last year.
Of course I use to this embed them, and find all my album art with this
EDIT: that explains why when I replaced the existing cover art, I never saw anything over 200x200