Good Morning everybody! (saying this around 18:00 makes me feel a bit weird though...)
Textures always come from the ERF-Files, they are in the TexturePack directories. For extraction purposes you can use the ERF-Tool for NWN, reinserting does not work right with it though.
The original Textures are not, I repeat, are not TGAs! Do not try to insert a TGA instead of another original texture into an ERF, it will not work! (in case of the "swpc_tex_gui.erf" KotOR will crash on you!)
I'm a (bad ?) programmer as well, I don't really have hand for art.
Maybe I'll throw out a quick utility for getting the sizes from the ERFs directly, so you can work with it a little easier. That wouldn't be a bad idea. Okay, I'll see what I can do.
Lil' Jawa :
I bet you wanna see the texture I used on her. Might be good for a grin.