Making custom textures / "skins"

[SixPoison]

Pretty much asking what the title says; how would I go about making custom textures? I'm a klutz with most technical things but I've made skins for games before and I'd like to try my hand at making some for Tr1. I have all sorts of graphical programs so I can save things in a correct format. Is this possible yet? If so, how would I go about doing this?