Try adding this CSS to your template (after saving a backup copy, of course).

.bx-content-wrapper {

        position: relative;

        /*--- This is where you can change the background to an image ---*/

        background-image:  url("YOUR-IMAGE-URL");

        background-size: cover;

        background-attachment: fixed;

        position: absolute;

        background-repeat: none;

}

I am not a professional code writer or CSS user.  I hope this helps!