You have to modify the model in 3DS Max. As you have to do with EVERYTHING that regards the model directly. As you have to with any other game =)
I suggest taking a look at the portraits. Though without a highpoly-torso, I wouldn't even start it up.
Still, I believe there is a portrait for the Tauren Marine, I think I saw it during a funmap, so you might want to check your Data Editor.
Quote from KratsAU:
Okay so my point stands, you can't make custom portraits from existing models ;(
You can. The same way you had to do it in WarCraft 3. You have to add a cam to the model and name it in the right way.