A good place to start is here:
http://forums.pocketplane.net/index.php?topic=52.0And here:
http://forums.pocketplane.net/index.php?topic=191.0NI really isn't enough. I recommend DLTCEP. I actually use both DLTCEP and Shadowkeeper for creating NPCs, along with WeiDU of course. You can assign a character a portrait in DLTCEP and get it into the game with WeiDU. Here's the code, which should go in your .tp2:
COPY ~MYNPC/Portraits/MYNPCS.bmp~ ~override/MYNPCS.bmp~
COPY ~MYNPC/Portraits/MYNPCL.bmp~ ~override/MYNPCL.bmp~