Untitled

#overlay{
background-image:url(../images/overlay.png);
width:100%;
height:100%;
margin:0px;
position:absolute;
z-index: 20;
}

#overlay {
background-image: url(../images/overlay.png);
width: 100%;
height: 100%;
margin: 0px;
position: fixed;
z-index: 20;
}

<body>
   <div id='wrapper'>
   </div>
   <div id='footerwrapper'>
   </div>
   <div id='overlay'>
   </div>
</body>