@OrcaellaSaurusIDPaint.net is like adobe photoshop. I recommend Gimp2 as well its a free program and does a good amount of the things photoshop does. Pretty sure it does more editing then paint.net. You will need to find some plugins called NormalMap, insanebump and bumpmap to make normal maps for any texture that you make. You will also need the dds plugin so you can load dds textures which is what gtav uses.