are you talking about the shut-down or the stand-by picture?
When you shut down the boox there is normaly no picture only if you enter the stand-by mode.
If you want an shut-down picture you can take any picture and name it shutdown.png. Then you put it in /usr/share/gui_shell/images
The stand-by pictures shoult ber named screen_saver0.png, screen_saver1.png and so on.
They should be put in \usr\share\system_manager\images