Yes I knew they hadn't designed the game with custom portraits in mind--though I think they should have--the portraits provided aren't all that various, and aren't a particularly close match for the face of the in-game character.
It funny, the BG and IWD series all provided for custom portraits,as did Arcanum. They seemed to be understandably disappearing because of the full 3D modeling of faces in games like Morrowind and Knights of the Old Republic. (NWN was a wierd compromise because portraits were provided but had nothing to do with the face of the character in game. It was genuinely distracting for me.)
Now BD is exactly the kind of game where I think custom portraits would work, and I always get a kick out of individualized my characters in this way.
(I do appreciate the amount of individuality larian has provided in body build, hair color, etc,)