title: Facebook's Default Image When `og:image` is Invalid
body: <p>How does Facebook choose a preview image when the <code>og:image</code> tag is invalid?  A friend and I were curious why a Facebook preview image didn't show up in the article — when we ran the URL through the Facebook debugger, it provided the following information.</p>
  
  <blockquote>
    <p>Provided og:image could not be downloaded or is not big enough. Please use an image that's at least 200x200px and is accessible from Facebook. Image '<a href="http://lostoregon.files.wordpress.com/2014/06/foodcarts.jpeg" rel="nofollow">http://lostoregon.files.wordpress.com/2014/06/foodcarts.jpeg</a>' will be used instead</p>
  </blockquote>
  
  <p>Tangent: Running the debugger once appears to clear Facebook's cache, and on the next run Facebook grabbed the correct (since fixed?) image. </p>
  
  <p>The one part of this I don't understand is: How did Facebook chose the <code>http://lostoregon.files.wordpress.com/2014/06/foodcarts.jpeg</code> image to use instead?  When I look at the <a href="http://lostoregon.org/2014/06/20/a-new-direction-for-lost-oregon/" rel="nofollow">original page</a>, <code>foodcarts.jpeg</code> isn't shown anywhere, which means either</p>
  
  <ol>
  <li><p>Facebook has some secret sause for chosing from images it's seen before</p></li>
  <li><p>Facebook picked an image that was on the page at the time, but is no longer there</p></li>
  <li><p>Something else I'm not considering</p></li>
  </ol>
  
  <p>Is this default image picking defined, deterministic behavior?  If so, how does it work?</p>