The .thumbnail class can be applied directly to an <img> element, or an <a> that wraps it. Make sure the <img> has an alt attribute that describes the contents of the image.

<img class="thumbnail" src="assets/img/thumbnail/01.jpg" alt="Photo of Uranus.">
<a href="#" class="thumbnail"><img src="assets/img/thumbnail/02.jpg" alt="Photo of Neptune."></a>
<img class="thumbnail" src="assets/img/thumbnail/03.jpg" alt="Photo of Pluto.">