You'll need max-width: 100% and position your background center center fixed.
It works best to put this on the html element as it's more reliably the full height of the browser. That said, body seems to be working fine for you here when I revised in the inspector.
body.custom-background {
background: url('http://test.justinhawleydesign.com/wp-content/uploads/2015/09/camera-green22.png') no-repeat center center fixed;
max-width: 100%;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}