根据设备方向更改背景位置

body {
  background: url(https://placehold.it/500x1000);
  background-size: cover;
  background-position: center center;
}

@media screen and (orientation: landscape) {
  body {
    background: url(https://placehold.it/1000x2000);
  }
}

@media screen and (min-width: 1200px) {
  body {
    background: url(https://placehold.it/2000x3000);
  }
}

<!-- body tag is included by default... -->

<!-- hey StackSnippet warriors(trolls)... key example here of a bad time to use it -->