I checked an alternate portratrait mod from Gibberling 3, and the files are called.
nJaheirL.bmp, nJaheirS.bmp, nJaheirM.bmp
nViconL.bmp, nViconS.bmp, nViconM.bmp
nAerieL.bmp, nAerieS.bmp, nAerieM.bmp
nImoenL.bmp, nImoenS.bmp, nImoenM.bmp
I don't know if it the way they must be called or if it is just G3's way of naming them.
You could try those names and see if they change the portraits for Viconia, Jaheira... and if they do, you can experiment with Valygar. nValyga, or nValyg...
Or someone who really knows how to do it might be able to tell you better. I never used any mods myself so don't know ho to do it properly