What do the gold and silver frames at the top left corner of in-game avatars represent?
It represents what raid-bracket they are in, i.e. how many cups they have.
The little corner on everyone’s icon indicates how many cups they have for Raid:
Bronze = 0-600 cups
Silver = 600-1200 cups
Gold = 1200-1800 cups
Platinum = 1800-2400 cups
Diamond = 2400+ cups
Ah OK. I should have realized that given my own status.