CSS background dinâmico

HTML

<body id="corpo">


<script>
  imagens = [ 'loremflickr.com/1920/960/brazil,rio',  'loremflickr.com/1920/960/dog', 'loremflickr.com/1920/960/paris' ]
  var i = 0;  
  corpo = document.getElementById("corpo");
	  setInterval( function() { 
	    corpo.style.backgroundImage = 'url(https://' + imagens[i] + ')' ;
	    corpo.style.backgroundSize = 'cover' ;
          if ( i < 2 ) {
            i = i + 1;
         }else{
	       i=0;
         }
      }, 3000);
</script>


</body>

CSS

body {
  background-image: url(loremflickr.com/1920/960/brazil,rio);
  background-size: cover;
}