NOTE: if you choose to do this it will need to be done at every workstation as the file that serves as the backdrop is in the local folder
The background images that show on Live are stored locally on each machine and are called backdrop_vebra.jpg and backdrop_vebra_WS.jpg (the widescreen version)
A copy of the appropriate one is copied by the system into C:\PSGLive\local\temp and named backdrop_system.jpg. This is the actual image that you see as your backdrop.
Changing these images will change the backdrop but attention does need to be paid to the screen resolution of the specific machine
On balance, unless it is crucial, it is probably easier to stay with the default background