body { 
  background: url(blue.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

.on {
  background-image:url('red.jpg');
}

.off {
  background-image:url('red.jpg');
}