Instructions
Open "My Computer" and browse to your "Baldur's Gate" install directory.
Create a folder labeled "Portraits" in the install directory if it does not exist already.
Place your custom portrait into the "Portraits" folder. Make sure your custom portrait is in the .BMP (bitmap) format, otherwise it will not display properly in "Baldur's Gate."
Launch "Baldur's Gate" and load your game. Click your character and click the "Character Record" button in the left menu bar.
Click the "Customize" button followed by "Appearance" and "Custom."
Select your custom portrait and click "OK."