html, body { 
  min-height: 100%; 
}

body {
  background-position: center center;
  background-attachment: fixed;
  background-repeat: no-repeat;
  background-size: cover;
}

@media (max-width: 480px) {
    body {
        background-image: url(923-small.jpg);
    }
}      

@media (min-width: 481px) {
    body { 
        background-image: url(923.jpg); 
    }
}