title: Give Liquid-templated <img> a class in Locomotive
body: <p>I have the following liquid markup:</p>
  
  <pre><code>{{ 'image.jpg' | theme_image_tag }}
  </code></pre>
  
  <p>and it renders like,</p>
  
  <pre><code>&lt;img src="/site.com/site/3424242/image.jpg" /&gt;
  </code></pre>
  
  <p>How to add a class or whatever option to it? I want it to render like:</p>
  
  <pre><code>&lt;img src="/site.com/site/3424242/image.jpg" class="thumbnail" /&gt;
  </code></pre>
  
  <p>I use the Locomotive CMS and the liquid that comes with it.</p>