body{
    width: 100%;
    background: url(../img/header.jpg) repeat-y center top fixed;
    -webkit-background-size: 100% auto;
    -moz-background-size: 100% auto;
    -o-background-size: 100% auto;
    background-size: 100% auto;
}

@media (min-width: 2000px) {
    body {
        width: 100%;
        background: url(../img/header.jpg) repeat-y center top fixed;
        -webkit-background-size: 2000px auto;
        -moz-background-size: 2000px auto;
        -o-background-size: 2000px auto;
        background-size: 2000px auto;
    }
}