1. All you need to do is get WC3 Image Extractor II Get it here:
http://www.wc3sear.ch/?p=Tools&ID=6
2. Then, using Image Extractor, find the skin you want and right click and save it as a BLP file.
3. Then go to File>Open. Open the skin you just saved.
4. Then go to Save>Image and save it as a TGA. Open the TGA you just saved in Photoshop, Paint Shop Pro, or whatever paint program you use.
5. Edit the skin however you want and save it.
6. Open your TGA in Image Extractor. Go to Save>Image and save it as a BLP.
7. Then, import your BLP into your map and use a custom path.
If you need anymore help on this PM me, email me, talk to me on AIM, or talk on MSN.