body {
  height: 100vh;
  background-size: cover;
  animation-name: slide;
  animation-fill-mode: both;
  animation-duration: 10s;
  animation-iteration-count: infinite;
}

@keyframes slide {
  0%,49% {
    background-image: url('https://picsum.photos/900/400/?random=1');
  }
  50%,100% {
    background-image: url('https://picsum.photos/900/400/?random=2');
  }
}