The background image comes from a zentools2 module assigned to the 'banner' position. In the quickstart package the module is titled 'Front Page Slideshow' and has an id of 463.
The module uses the 'slideshow-backstretch' layout and Joomla articles as a content source.
It uses 2 elements in the layout, image and title.
Under the layout and elements are the general slideshow options. The height is set to 600px.
In the content tab, the number of items is set to 1 and the folder that contains the image to be used is selected.