Hello,

Sorry for the late reply.
Please add this code into your custom css box:

footer#footer .inner {
    background-image: url(https://codeless.co/specular/default/wp-content/uploads/2014/07/1231-compressed.jpg);
    background-attachment: fixed;
    -webkit-background-size: cover;
    background-size: cover;
    background-repeat: no-repeat;
    overflow: hidden;
    background-position: 50% 0;
    box-sizing: border-box;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    position: inherit;
}

Edit the image url, to the one you want to use.
Let us know if you need further help.

Best regards!