title: How does facebook obtain URL metadata to display as preview in shared links?
body: <p>I just noticed a shared link on facebook whose preview had an image that was not contained on the page itself (at least not visibly). So I'm wondering -- is there any way of specifying what images can be shown in the thumbnail/preview when a link is shared using HTML code? </p>
  
  <p>The only way I can think of doing this would be to have a 0px image on the page that can be used in the preview. Is this what's usually done?</p>