How to enter custom portraits for non-playable NPCs and enemies?

http://forum.baldursgate.com/discussion/8568/come-usare-ritratti-portraits-personalizzati (This page is in Italian)

HI, I saw the page to insert the custom portraits to the NPC who join the party, but what are the codes to be entered to customize the npc also important non-player characters and enemies as Sarevok, etc ...?