It's not an actual image, it's saved in CSS which is why right-clicking won't work. Here's the JS you'll need to run:
prompt("Here's your background image!",getComputedStyle(document.getElementsByTagName('body')).getPropertyValue('background-image'))
To run it, go to the console (either right-click and press Inspect Element for Firefox or Chrome, or bookmark Firebug Lite and run it on IE or Safari). Now go to the "console" tab and run the above script. Note that the url will be surrounded by url() like url(myimage.png).
"A window.prompt() dialog generated by this page was suppressed because this page is not the active tab of the front window. Please make sure your dialogs are triggered by user interactions to avoid this situation."