should be easy:
• open the ???_default.jpg, edit/selection->select all, copy
• open your skin, (paste as->new layer) / (create new layer->paste), probably resize this layer
• move this layer on top of all other layers, select mixing-mode (don't know how exactly it is called, usually a dropdown-list where you can find things like "darken", "lighten", ...) "multiply"
now you should have the default-skin for shading. should work with all "better" graphic editors like gimp, Paint Shop Pro, Photoshop, ...