Fen employs two bust artists, Adjatha and Shou, and they produce the vast majority of the busts in the game. Occasionally additional busts are commissioned either by the writers or by Fen himself.
They're on a payroll. Fan busts can get in if they're of acceptable quality, but there's no hard rule or anything on that. I don't know who Velta is but if she has no bust she might be in the list for one already.
