lazyLoad.js

function lazyLoadImages() {
  var lazyImages = document.getElementsByTagName('img[data-src]');

  for (var i = 0; i < lazyImages.length; i++) {
    if (!lazyImages[i].getAttribute('data-src')) {
      return;
    }

    var imageUrl = lazyImages[i].getAttribute('data-src');
    lazyImages[i].setAttribute('src', imageUrl);
  }
}

window.addEventListener('load', function() {
  // ...

  lazyLoadImages();
}, false)