You can use Fancybox for this http://fancyapps.com/fancybox/
First you need to load jQuery and fancybox to your page:
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/fancybox/2.1.5/jquery.fancybox.pack.js"></script>
After add link to the large image around your small image
<a href="http://www.yoursite.com/large-image-name.jpg" class="various"><img src="http://www.yoursite.com/small-image-name.jpg" /><a>
now you need to run fancybox
jQuery(document).ready(function() {
jQuery(".various").fancybox({
maxWidth : 450,
maxHeight : 350,
fitToView : false,
width : '70%',
height : '70%',
autoSize : false,
openEffect : 'none',
closeEffect : 'none'
});
});