var bg_im;
var mw = 1920;
var mh = 1080;
var ratio = mw/mh;

function updateSize()
{
        if (typeof bg_im == "undefined")
               bg_im = document.getElementById("bg_container");
        var size = getWinsize();
        if (size.h > (size.w / ratio))
        {
               bg_im.style.width = Math.ceil(size.h*ratio) + "px"
               margin = "0 0 0 "+ Math.round((size.w-(size.h*ratio))/2) +"px";
               bg_im.style.margin = margin;   
        }
        else
        {
                       bg_im.style.width ="100%";
                       bg_im.style.margin = "0";      
        }
}

function getWinsize()
{
        var _obj = new Object();
        if( typeof( window.innerWidth ) == "number" ) 
        {
        //Non-IE
               _obj.w = window.innerWidth;
               _obj.h = window.innerHeight;
        }         
        else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) 
        {
    //IE 6+ in 'standards compliant mode'
               _obj.w = document.documentElement.clientWidth;
               _obj.h = document.documentElement.clientHeight;        
        } 
        else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) 
        {
    //IE 4 compatible
        _obj.w = document.body.clientWidth;
        _obj.h = document.body.clientHeight;
        }
        return _obj;
}