do not modify is hardly ever seen, the background image for the control panel will show BUT you must make sure that the controlpanel background is exactly the same colour as the windows background colour, if it is not, it will not show up.
you will have to be very precise and get it correct. you can put whatever image or design you want on top of the control background image (for example.... a shape of a spanner), but just make sure that its background is the same colour as the windows background colour.
the windows colour by default is white, go to change skin colours (which is just above the change skin fonts) tab and
you will see an explorer box that says " show me the skin colour presets" click on the arrow beside it and choose the option below which says: "show me the skin defined system colours"
this is where you see your window colour and its called " Window background" this is the colour you have to match with the control panel background image. if not, it wont show up.
hope this helps.