It is quite easy. Download timbthumb and upload to your ftp (e.g: your root folder)

Then open your template html. For default template; templates/partials/assets/images.html

<strong>find</strong>
<code>


</code>

<strong>change like</strong>

<code>
<a href="@url">
</a>

</code>

p.s: dont forget to add shadowbox requirements. otherwise shadowbox wont work.