I suggest you use Warcraft3 Viewer, it's in the DL section,in the program: choose; convert files >> BLP -> TGA, BMP or JPG, I'd choose BMP.
Then open the skinning program your using (I prefer Photoshop), open the converted *.bmp file of your skin and start to edit it!
I don't know if this answered your question, but it might've helped.
Although, if you're using paintshop, I don't know.
In the Import Manager, Double click on the File, and check "use custom path".
Also, use the War3Extractor 2 to get .blp s into .tga s for editing, and then use it again to open the .tga, and save the .tga as a .blp