Hello,
That option will work only if you select site layout “Boxed” in Theme Options > General.
Now you need add this code in Theme Options > Custom CSS > Custom CSS for desktop:
.st-content {
background-image: url("https://www.thermalprinteroutlet.com/wp-content/uploads/2015/09/background.png");
background-repeat: no-repeat;
background-position: center top;
background-size: cover;
}
Please note your image is too small in height and it will be distorted on your site. We recommend you to edit its sizes.
Regards,
Eva Kemp.