http://forum.toribash.com/showthread.php?t=63013

You create 128 pixel by 128 pixel textures using your favorite imaging program (Like Photoshop, Gimp, or Paint.Net) save them as .tga; in the uke folder (Where Toribash is installed, find custom, then find uke) and name them properly (See the thread above)

More formats for textures are available here: http://forum.toribash.com/tori_textures.php

Now here's the cool part, if you get good in it you can sell textures in the Art/Textures Market Thread!