var viewportwidth;
var viewportheight;
// the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight
if (typeof window.innerWidth != 'undefined'){
	viewportwidth = window.innerWidth,
viewportheight = window.innerHeight
}
// IE6 in standards compliant mode (i.e. with a valid doctype as the first line in the document)
else if (typeof document.documentElement != 'undefined' && typeof document.documentElement.clientWidth !=	'undefined' && document.documentElement.clientWidth != 0){
viewportwidth = document.documentElement.clientWidth,
viewportheight = document.documentElement.clientHeight
}
// older versions of IE
else{
viewportwidth = document.getElementsByTagName('body')[0].clientWidth,
viewportheight = document.getElementsByTagName('body')[0].clientHeight
}
if(viewportwidth < 801) document.body.style.backgroundImage="url('img/bg_800.jpg')";
else{ 	if(viewportwidth < 1025) document.body.style.backgroundImage="url('img/bg_1024.jpg')";
			else{ 	if(viewportwidth < 1281) document.body.style.backgroundImage="url('img/bg_1280.jpg')";
						else{ 	if(viewportwidth < 1601) document.body.style.backgroundImage="url('img/bg_1600.jpg')";
									else document.body.style.backgroundImage="url('img/bg_2000.jpg')";
						}
			}
}