well, in the cartexture folder, make a copy of the texter for the kind of texture you want, rename it, and edit it so that it looks the way you want it to.
Actually, both can be either tga or png, it doesn't matter, the game can load both. But as for paint, it can load png's but no tga's, you'll need a converter for that otherwise, you should have photoshop or paint shop pro.