That all depends it does if I try it here. Remember if you do not allow guests access to see the photo how would the facebook share script be able to get the image? Just an observation on my part. That is the general rule that alot forget is if you lock down access to guests they forget that they are logged in and viewing a photo where as others can not.
Now there are new open graph meta tags that can be added to the page that might help.
You can edit pp-inc.php and on the globals line for the function printheader add $imgurl. Once you do that you can lower in that function where noted below add the code in bold and see if that helps you.
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