Hi there, you can add your own background image to the site using css. It depends really where you want the image as the page is split into sections etc.
The main section which has all the page content and is underneath the slider is called #container. So you’d need to upload your image into the media tab and get the url for the image.
Then add the following css, adding in your image path/url into your style.css at the bottom and save.
background-image: url("your image url goes here.jpg");
Hope that helps.