You can use any kind of image editing program you want.
Photoshop is a popular one, so is
Paint Shop Pro. Neither of those two are free, however. You'll find a good freeware image editor in
the GIMP. Hell, you can even use MSPaint if you want; there's no specialized tool you have to use.
The way you apply the skin to your model, however, depends on what modelling program you're using. It's done in that, and not your image editor.
And if anyone tries to steal your vertices again, make sure you let us know. :O