to get the wallpaper behind the icons to change when you theme it you need to ssh the touch or get the filebrowser from installer and find the file named "lockbackground.jpg" and delete it.
you need summerboard and customize installed and setup like this
summerboard prefs --
turn everything off but the wallpaper. and select customize as theme.
and turn on the wallpaper in the customize prefs..
hope that helps does it for everytime !@!