html {
  width: 100%;
  height: 100%;  
  position: fixed;
  top: 0;
  left: 0;
  z-index: 0;
  background: url('tree.jpg') no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}