[SpiritWarrior]

This is from the BG readme, but the method is the same.

Custom 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.