One place to put em is
http://www.wowinterface.com/downloads/cat131.html
I don't think Spiel has any specific rules about skins, just be aware that less then skinny clothed people as a background may be against he TOS of this site, and perhaps any to bloody looking art perhaps.
As for updating, if you mean WoW patches, changing the toc should be enough, if nUI then unless you go from nUI5 to nUI6 I don't think there is any need to change things.
Kinda like the little touches you did on the screen shot. Keep it up.