Lozad.js

<script src="https://cdn.jsdelivr.net/npm/lozad"></script>
  <img class="lozad" data-src="01.jpg" />

// Initialize library
lozad('.lozad', {
    load: function(el) {
        el.src = el.dataset.src;
        el.onload = function() {
            el.classList.add('fade')
        }
    }
}).observe()