html {
   min-height: 100%;
   background-image: url(../img/istock-875722302-1920.jpg);
   background-size: cover;
   background-repeat: no-repeat;
   background-position: center center;
}

/*
@media only screen and (max-width: 767px) {
  html {
     background-image: url(smaller-image.jpg);
  }
}
*/