<style>
img { opacity: 1; transition: opacity 0.3s;}img[data-src] { opacity: 0;}
</style>
<img data-src="/path/to/image.jpg" alt="">
<script>
[].forEach.call(document.querySelectorAll('img[data-src]'), function(img) { img.setAttribute('src', img.getAttribute('data-src')); img.onload = function() { img.removeAttribute('data-src'); };});
</script>
