JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://jsxt.googlecode.com/svn/trunk/js/String.js"></script>
<div class="thumbnail">
<img src="thumbnail_1.jpg" />
</div>

<div class="thumbnail">
<img src="thumbnail_2.jpg" />
</div>

JavaScript

$('.thumbnail').each(function(index, el) {
    $('<span/>', {
         'class': 'index',
         text: "%03d".sprintf(index + 1)
    }).prependTo(this);
});