Предзагрузка картинки для background

document.getElementById('bgpreload').addEventListener('load', () => 
  document.body.classList.add('bgLoaded') 
);

body.bgLoaded { 
  background-image: url("http://lorempixel.com/400/200/"); 
  background-color: red; 
}

<link id="bgpreload" rel="preload" as="image" href="http://lorempixel.com/400/200/" />

var img = document.createElement('img'); 
img.onload = function() { 
  document.body.className += ' bgLoaded'; 
} 
img.src = 'https://placeimg.com/400/200/any';

body.bgLoaded { 
  background-image: url("https://placeimg.com/400/200/any"); 
  background-color: red; 
}