html, body {
height: 100%;
margin: 0;
padding: 0;
}
img#bg {
position:fixed;
top:0;
left:0;
width:100%;
height:100%;
}

#center {
  position:relative;
  z-index:1;
  top: 50%;
  margin-top: -100px;
  width: 390px;
  height: 250px;
  left: 50%;
  margin-left: -195px;
}