You can past the picture in your custom folder, if it has the correct name for the joint.
After this you'd also need to enable your joint textures in the item.dat file, there's a complete item.dat tutorial over here http://forum.toribash.com/showthread.php?t=195093
But I don't know whether joint texturing isn't included in there, as I'm pretty sure joint textures weren't released in 2010 yet. I hope someone else could tell something more about the item.dat part, but It shouldn't be so hard.