It needs to be a certain kind of file, a certain size, and named in a certain way. Let me find a website or three...
http://www.geocities.com/drizzt_darkelf2001/ http://www.tgeweb.com/ironworks/bald...ortraits.shtml http://www.nextrpg.com/bg2-custom-portraits.php
Taken from Drizzt's Portraits: "The measurements for player portraits are 38X60 pixels for the smaller portraits (8-bit color runs faster but it can be up to 24 bit color), and 110X170 pixels for the larger portraits. To use a custom portrait, place it in the Portraits directory (in the install directory for the game) and add an S to the end of the small portrait name and an L to the end of the large portrait - otherwise the two names must be identical. The names must be 1-7 letters long, maximum - so the filename with the S or L added to the end is no longer than 8 total characters. The portrait files must be in BMP format.
For example:
XXXXXXXS.bmp for small (38x60, 8-bit)
XXXXXXXL.bmp for large (110x170, 24-bit)
For multiplayer, each player must have a copy of the portraits, otherwise they will see a blank icon instead of the other player's custom portraits. You will need to manually send these portraits to your friends as BG2 has no method of transferring them."
How to... 
Linear Mode
