Hello,

You can add it by using custom css code. Example, add this code itno your custom css box:

footer#footer .inner {
background-image: url("https://104.131.255.213/specular/onepage/wp-content/uploads/2014/09/shutterstock_189386465-compressed.jpg");
    background-color: rgb(34, 34, 34);
    background-size: cover;
    background-position: center center;
    background-repeat: no-repeat;
}

Change the values to your wish.

Best regards!