data.chinahighlights.com/js/image-loader-1.0.js

function createImageLoader(settings) {
  var loadImageByUrl = function(target) {
    target.addEventListener("load", function() {
      target.style.opacity = 1;
    });
    target.setAttribute("src", target.getAttribute("data-src"));
  };
  
  if (!Array.from) {
    return function from(arrayLike) {
      console.info(arrayLike);
      return [].slice.call(arrayLike)
    }
  }
  var imageList = [].slice.call(document.querySelectorAll('img[loader=ready]'));
  imageList.forEach(function(element, index) {
    element.style.opacity = 0;
    element.style.transition = "opacity 0.5s ease-in-out";
    loadImageByUrl(element);
  });
  var lazyloader = createLazyloader();
  lazyloader();

  function throttle(fn, delay, atleast) {
    var timeout   = null,
        startTime = new Date();
    return function() {
      var curTime = new Date();
      clearTimeout(timeout);
      if (curTime - startTime >= atleast) {
          fn();
          startTime = curTime;
      } else {
          timeout = setTimeout(fn, delay);
      }
    }
  }
  function getOffset(el) {
    var box = el.getBoundingClientRect();
    return {
      top: box.top + window.pageYOffset - document.documentElement.clientTop,
      left: box.left + window.pageXOffset - document.documentElement.clientLeft
    };
  }
  function createLazyloader() {
    var lazyImageList = [].slice.call(document.querySelectorAll('img[loader=lazy]'));
    lazyImageList.forEach(function(element, index) {
      element.style.opacity = 0;
      element.style.transition = "opacity 0.5s ease-in-out";
    });

    return function() {
      var seeHeight = window.innerHeight;
      var scrollTop =
        (document.documentElement && document.documentElement.scrollTop) ||
        document.body.scrollTop;

      lazyImageList = lazyImageList.filter(function(element, index) {
        var offsetTop = getOffset(element).top
        var hasVisible = offsetTop < seeHeight + scrollTop;
        if (hasVisible) {
          loadImageByUrl(element);
        }

        return !hasVisible;
      });
    }
  }

  if (window.addEventListener) {
    window.addEventListener('scroll', throttle(lazyloader, 500, 500), false);
  } else if (window.attachEvent) {
    window.attachEvent('onscroll', throttle(lazyloader, 500, 500));
  }
}