title: Twitter Bootstrap responsive utility class usage
body: <p>How are the Twitter Bootstrap responsive utility classes such as .visible-desktop supposed to be used. I have tried to follow the example on the Bootstrap web site, </p>
  
  <pre><code>&lt;span class="visible-desktop"&gt;✔ Desktop&lt;/span&gt;
  </code></pre>
  
  <p>on an image like this,</p>
  
  <pre><code>&lt;span class="visible-desktop"&gt;
    &lt;img src="&lt;%=(image_path('green.jpg'))%&gt;"&gt;&lt;/img&gt;
  &lt;/span&gt;
  </code></pre>
  
  <p>but the image remains when I shrink the browser window to tablet or phone size.</p>