Hmmm The NiMaterialProperty only changes the ambient color, diffuse, specular colors, etc... To change textures, you must look for NiTextureProperty which has NiImage. There you can edit the the external link to the TGA texture. Other then that, nifksope doesn't edit the textures like photoshop, etc. The newer versions of nifskope can edit the UV in case there's a simple mapping error.
Should I still not understand what you are talking about, lol, you can catch me on WLM (MSN)