thing.html

<li id="1">
  <div><img data-src="img1.png"/></div>
  <div><img src="img2.png"/></div>
<li>

<script>
  jQuery(function() {
    setInterval(function() {
      // select a random slot to work with. For that slot:
      // 1. Ensure that background image `src` attribute is set (load the image)
      // 2. Once image is loaded, fade out foreground div (opacity: 0)
      // 3. Move foreground div to background
      // 4. Change its opacity back to 1 for the next time it's switched
    }, 1000);
  });
</script>