.sd {
  width: 100%;
  height: 50%;
  overflow: hidden;
  border: 10px solid;
  border-top-color: #76b729;
  border-left-color: #76b729;
  border-bottom-color: #76b729;
  border-right-color: #76b729;
  position: relative;
}

.sa {
  position: absolute;
  animation: round 16s infinite;
  opacity: 0;
  width: 100%;
  height: 100%;
}

@keyframes round {
  25% {
    opacity: 1;
  }
  40% {
    opacity: 0;
  }
}
img:nth-child(1) {
  animation-delay: 12s;
}

img:nth-child(2) {
  animation-delay: 8s;
}

img:nth-child(3) {
  animation-delay: 4s;
}

img:nth-child(4) {
  animation-delay: 0s;
}