title: How to get a YouTube thumbnail from a YouTube iframe?
body: <p>For instance I have a blog post that has the following <code>iframe</code>.</p>
  
  <pre><code>&lt;iframe width="420" height="315" src="//www.youtube.com/embed/1sIWez9HAbA" frameborder="0" allowfullscreen&gt;&lt;/iframe&gt;
  </code></pre>
  
  <p>How can I extract the thumbnail from this <code>iframe</code>?</p>