https://betterproposals.io/hosted_scripts/lazyload.js

1(function() {
2    var elements = document.querySelectorAll('img[data-src]');
3    var index = 0;
4    var lazyLoad = function() {
5        if (index >= elements.length) return;
6        var item = elements[index];
7        if ((this.scrollY + this.innerHeight) > item.offsetTop) {
8            var src = item.getAttribute("data-src");
9            item.src = src;
10            item.addEventListener('load', function() {
11                item.removeAttribute('data-src');
12
13            });
14            index++;
15            lazyLoad();
16        }
17    };
18    var init = function() {
19        window.addEventListener('scroll', lazyLoad);
20        lazyLoad();
21    };
22    return init();
23})();