In troubleshooting textures and Nifskope I do a few things
1. drag a texture that you know works (any texture will do - as long as you know it is showing up in game) into the same folder as where you have your new texture
2. open up the mesh in nifskope
3. drag the texture you know that works onto nifskope (which automatically detects the filepath)
- if no texture shows up on the mesh you now know you have a filepath issue - with Win 7 I get this a lot and it took a lot of fiddling for me to get Nifskope to read all the filepaths
- if the texture does show up on the mesh then you have eliminated filepath being the problem
4. Now drag your new texture onto the mesh - if it shows up white then you have an incorrect format or incorrect size
format is dds, bmp, or tga
size is any combination of 32, 64, 128, 256, 512, 1024, 2048 eg 512x512 or 128x1024
Sometimes when making a texture I'll add a shadow to a layer - this almost always changes the image size and unless I resize back to 512 or 1024 in my final image I will get the whiteout that you describe in your OP - it makes me mad too