Von's way should work. However, in my experience, you also need to put the new portraits in your 'portrait' folder.
If you do a search for the different portraits around, you will find that there are more portraits than you could've imagined. IMO, there are a couple good sites around, such as PXG's portraits
and Divine Treasures
. Nowadays, I use almost none of the original BG portraits. I've switched them all for ones that are more to my taste.
You can simply download these portraits, save them, and (like Von mentioned) change their names to to the respective character. If you don't know a characters exact name, for example, valygar's name is too long for the portrait name, you can easily look it up with SK.