go to your registry editor:
HKEY_CURRENT_USER\Software\A_C\S2U2
Go to Wallpaper and change the file name of the wallpaper that you want. I tried "\Program Files\S2U2\gfx\battery1.png" without the quotes just to make sure it worked and it does. You may have to have to hit the iLock2 to get it working.
I would edit the battery for you, but you don't want to see my MS Paint skills. Here are the battery files though, and the newest S2U2. which can be found
here
Edit: battery4.zip is my feable attempt. To use this, save the png to the gfx folder and rename it in the registry "\Program Files\S2U2\gfx\battery4.png"