If you want the background to update automatically, you have to create a linked background. There is a limitation that a linked background can only be added by URL. You cannot use a local file as a linked background.
Is it possible for you to save your .png to a web site instead so that you can create a http://server/mypicture.png linked background?
http://www.solarwinds.com/netperfmon/SolarWinds/OrionNetworkAtlasAGAddingBackground.htm