You can add it using CSS, you just need a way of adding your own CSS since the theme doesn’t have a custom CSS option. One way is to create a child theme but it’s easier to use a CSS plugin like Lazyest Stylesheet or Custom CSS Manager (I prefer Lazyest Stylesheet). Just make sure your image is light enough so it doesn’t interfere with the text. Or if your image is dark, you can change the background color of your container elements, maybe adjusting the opacity so the background image still shows through a little. If your image is large, you have to decide if you want the image to scroll when the user scrolls the page down or remains fixed in place (background-attachment property). If your image is small, you can tile it using the background-repeat property.
