Is it possible to set-up Apache so that all sites served from the
installed version, have an identifying image overlaid?

I have 2 installs of WAMP (one local and one internal development).
Sometimes it can be easy for both myself and testers to be looking at
the wrong version of the server. Is there a way to overlay a PNG (like
the 'Make poverty history' banners) on anything served from a
particular instance of Apache?