html { 
  background: url(/images/crowd.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}
img {
   position: absolute;
   top: 50%;
   left: 50%;
   width: 600px;
   height: 250px;
   margin-top: -125px; /* Half the height */
   margin-left: -300px; /* Half the width */
}