title: Center an image inserted with after: pseudo-element; can't use spans
body: <p>I'm pulling in a tumblr feed using tumblr's code, and using after: to add an image as a separator between posts.  I'd like to center the image, but haven't had luck doing so.  Since tumblr's generating the content, not me, I don't think I can use span tags, which seems to be the usual answer.  Any other ideas?</p>
  
  <p>Page showing feed in use: <a href="http://www.lumn.net/index.shtml">lumn.net/index.shtml</a></p>
  
  <p>CSS:</p>
  
  <pre><code>.tumblr_post:after {
      content: url(../img/flower.png);
      display: block;
      position: relative;
      margin-top: 42px;
      margin-bottom: 24px;
      margin-left: auto;
      margin-right: auto;
  }
  </code></pre>