/* Remove this if you dont want background overlay over the image or adjust per your needs */
body.login_admin:before {
    content: " ";
    background: rgba(31, 31, 31, 0.7);
    min-height: 100%;
    min-width: 100%;
    position: absolute;
}
body.login_admin {
  background: url('https://affenkaefig.info/wp-content/uploads/2017/08/akfestival-9104.jpg') no-repeat center center fixed; /* The full image url*/
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}