body {
  background-image: url("bgr");
  background-repeat: repeat;
}
.monster {
  width: 231px;
  height: 341px;
  background: url('sprite_small.png') left center;
  animation: play .8s steps(20) infinite;
  position: fixed;
  top: 50%;
  left: 50%;
  /* bring your own prefixes */
  transform: translate(-50%, -50%);
}
@keyframes play {
    100% { background-position: -4620px; }
}