Havent played with the game for quite some time so i might get this wrong, but i believe its under texture > widetexture. Few things to note is that the texture come in dozens of splice, so you need to stitch it back together and after doing that you'll notice that the image is flipped upside down so you need to flipped it again. After youve done painting your new texture and you want to save, you need to flipped the image upside down, splice it and make sure to have the correct name before saving it. Thats the gist of it, i believe theres tons of map making software out there you can use to ease the process