In Ekklesia > top right links > click 'Easy Edit' > when your preview site opens, expand the Color Picker > Body Background

To edit the body background image view your homepage with the Easy Edit turned on, and open the Color Picker. The top button allows you to 'Choose File,' or you can 'Remove Image,' to display the color selected as the Dark Background Color item from the list below.

The Body Background image that you select will display behind the rotator and text with a parallax/page scroll effect when viewing on a desktop view.  This means the background image is fixed and the the interior content will scroll over the background image as the user moves down the page. 

There is not a specific image size requirement, but a recommended size would be ~1400 x 700 pixels.  The width works best with an image that is at approximately 1400 pixels, and the height can vary. Make the file size ~200KB.