Important: This trick relies on the user's browser having CSS3 support. We'll be building this into the core to support all browsers, but until then, if you're confident the majority of your visitors are using one of the following browsers, then read on..
To make it scale fullscreen:
Assign an image as the background image for your page (not a page section) by clicking on the "Change" button in the "Background Properties" panel.
Click on the "Stylesheets" button in the bottom left of the editor.
That's it! Save and republish. Note that you will not see the scaling effect take place inside the editor. Also note that there are different ways of scaling and positioning your background image. This example uses "cover" but you can read more about the background-size rule here...