title: Facebook shared grey rectangle instead of proper image
body: <p>I spotted that one (just one, rest is ok) shared link on wall of group I am in isn't look properly. It's just grey rectangle, you can see it on included image. I wonder why it's displayed this way because I copied url and pasted it to facebook open graph object debugger. There are no warnings and preview generated by tool is ok (so image pass dimension requirements). I also tried to put that url on my profile and I can see image. Sorry that I don't paste urls here but I don't think I'm allowed to do this. I hope someone still want to answer my qestion. What is reason of this behaviour?</p>
  
  <p><a href="http://i.stack.imgur.com/xvh0F.png" rel="nofollow">Grey rectangle on shared link</a></p>