display-image-after-loading.js

var img = new Image();
    img.src = 'img/assets/background.png';

var int = setInterval(function() {
    if (img.complete) {
        clearInterval(int);
        document.getElementById('content').setAttribute('style', 'background-image: url(' + img.src + '); opacity:1;');
    }
}, 50);

display-image-after-loading2.css

.content {
  //visibility: hidden;
  opacity:0;
  transition: opacity 400ms ease;
}