Hmm... I'll just post my message to him.
I assume that you have a model and skin, say a footman, from the game you want to tweak. First you need a viewer program of some sort. I prefer Warcraft 3 Viewer but there are others like Image Extracter (both in the DL section of this site) I will walk you through how to do this with WC3Viewer.
1. Open the viewer (Warcraft 3 Viewer)
2. Go to File>Open MPQ and choose war3.mpq (this is the file that hold all the skins/models/level/etc. for the origional game, not the expansion. This is important because the origional game has the footman file not the expansion. If, for example you wanted to skin Kael from FT then you would open the war3x.mpq) Hit OK.
3. Now that you have war3.mpq open, look in the box that lists the contents of that .mpq file. Go to this path by clicking through each level of files:
4. You should now see a footman in the blue window. Go to Current File>Extract (and convert) used textures. In the dialogue it list the various textures that are used in the model and what you want to do with them. You should see both Textures\Footman.blp and Textures\Guts.blp. The guts file is just the corps you see when the unit dies so it is not nessessary skin it.
5. Unhighligt the guts file so that only the Textures\Footman.blp is highlighted. In the obtions choose a location to extract the skin to. The desktop is an easy place to find it. Make sure that it is set to extract as a .tga file (look for a box in the lower right side)
6. Now open Photoshop and open up the file you just extracted from where ever you put it.
7. You can now edit the skin as you see fit.
*note* Some versions of Photoshop were not shipped with the targa plugin (lets you open .tga's). You can download it here