lazyloading.js

<script>
function isInView(el) {
    return (el.getBoundingClientRect().top >= 0) && (el.getBoundingClientRect().bottom <= window.innerHeight);
}

function checkVisibility() {
	var images = document.getElementsByTagName('img');
	for (var i=0; i<images.length; i++) {
		if(images[i].getAttribute('data-src') && images[i].getAttribute('src') == null) {
			if (isInView(images[i])) 
				images[i].setAttribute('src', images[i].getAttribute('data-src'));
		}
	}     
}
window.addEventListener('scroll', checkVisibility);
window.addEventListener('load', checkVisibility);
</script>