Make Your Monitor Invisible (well Almost)

Create a desktop background to amuse and confuse your friends and co-workers using just a digital camera and a bit of fiddling.

A computer system, a digital camera and some local currency

Remove your primary monitor and balance precariously in an unsuitable place.

Now photograph the view that a user of the computer would see if they looked through the monitor. (The local currency can be added at this point for side-splitting hilarity)
This step needs to be done very carefully for maximum effect, sitting on the chair usually used at the computer when taking the photo ensures the angles are correct.

Manipulate the colour, size and position of the image until it looks like the view through the monitor. A good program for this is the open-source program 'The Gimp'.
Now set the image as your desktop background, sit back and wait for the humour to happen.