1) Go to your User(s) folder. You know, the one that has My Pictures, My Music and so on. Look for My Documents, then Baldur's Gate - Enhanced Edition.
2) There you create a folder called Portraits (with a capital P), and that's where you placed your custom portraits. Mind that the dimensions are different now:
Large: 210x330 (not really necessary, so don't worry if you don't have it)
Medium: 169x266
Small: 54x84
Now you will notice that when choosing your portrait, a CUSTOM button appears right beneath the portrait you're currently seeing.
Also remember that file names cannot have spaces, need to be 8-chars long at maximum, and also require the letter S, M or L, according to their size. Ex;: xxxL.bmp, xxxM.bmp, xxxS.bmp
And yeah, they have to be bitmaps.
Also here is a nice site for portraits (Click ->
Link) But you will need to re-size them to the sizes stated above.
[Edited by ServiusTheBear, 12/1/2012 9:57:40 AM]