title: SweetAlert's image doesn't appear
body: <p>It's my first question. I hope to be clear.
  So, I use SweetAlert for the loading message with image, but the image doesn't appear and I don't know why...</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>window.onbeforeunload = function() {
    swal({
      title: 'Sweet!',
      text: 'Modal with a custom image.',
      imageUrl: 'https://unsplash.it/400/200',
      imageWidth: 400,
      imageHeight: 200,
      animation: false
    })
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdn.rawgit.com/t4t5/sweetalert/32bd141c/dist/sweetalert.min.js"&gt;&lt;/script&gt;
  &lt;link href="https://cdn.rawgit.com/t4t5/sweetalert/32bd141c/dist/sweetalert.css" rel="stylesheet" /&gt;
  
  &lt;a href="https://www.google.com/"&gt;Visit google&lt;/a&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>And I have just the frame of the image (as in the image):
  <a href="https://i.stack.imgur.com/nL3ZY.png" rel="nofollow noreferrer">enter image description here</a></p>
  
  <p>Help me please</p>