[Adam H]

I would imagine you would need to do something like :

html {
    background-attachment: fixed;
    background-color: transparent;
    background-image: url("images/background.jpg");
    background-position: 0 0;
    background-repeat: no-repeat;
    background-size: 100% auto;
}

Something like that should work, Havent tested it but you should be able to tweak it from there.