You can use any kind of graphics program that will handle bitmaps, I personally use Adobe photoshop, but that program is rather expensive. With SimPE, have a look at the mts2 and the SimPE site for their tutorials, also I think there may be some tutorials on this site which may help.
So, I can draw what I want in the skins with adobe photoshop, right? That's cool!
Thanks again wirelessguy.