body{background:url(https://www.gxlcms.com/bg.jpg) center center;background-size:cover;height:900px;width:100%; 
filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src='https://www.gxlcms.com/bg.jpg', sizingMethod='scale');} 

$(function(){ 
 $("body").append(""); 
 $("#main_bg").append(""); 
 cover(); 
 $(window).resize(function(){ //瀏覽器窗口變化 
 cover(); 
 }); 
}); 
function cover(){ 
 var win_width = $(window).width(); 
 var win_height = $(window).height(); 
 $("#bigpic").attr({width:win_width,height:win_height}); 
}