var scrollSpeed = 50;
    var step = 1;
    var current = 0;
    var imageWidth = 1500;
    var headerWidth = 900;        
    var restartPosition = -(imageWidth - headerWidth);
    function scrollBg(){
        current -= step;
        if (current == restartPosition){
            current = 0;
        }

        $('body').css("background-position",current+"px 0");
    }
    var init = setInterval("scrollBg()",scrollSpeed);