function bg_img_resize() 
{
    var w = $(window).width();
    var h = $(window).height();
    var iw = $('#bg img').attr('width');
    var ih = $('#bg img').attr('height');
    var rw = iw / ih;
    var rh = ih / iw;
    var sc = h * rw;
    if (sc >= w)
    {
        nh = h;
        nw = sc;
    }
    else
    {
        sc = w * rh;
        nh = sc;
        nw = w;
    }
    $('#bg img').css({height: nh, width: nw});
}

$(document).ready(function(){ bg_img_resize(); });
$(window).resize(function(){ bg_img_resize(); });

$(window).resize(function()
{ 
    bg_img_resize(); 
    $('#bg').css({ 'top' : 0, 'left' : 0 }); 
});

$(window).scroll(function()
{ 
    bg_img_resize(); 
    $('#bg').css({ 'top' : 0, 'left' : 0 }); 
});