I use on several of my sites this plugin. By some of those, if I publish a blog entry, it is published on Facebook Page, but no photo. The placeholder is a empty square. I have looked at the resulting Facebook code, but there is a long reference with the reference to the site inside it. This reference to the upload part is valid and readable.
I have compared the code if this site to one that is (still) working and I cannot find any differences. Also the parameters in Developer.Facebook seems to be OK. And older (before I think december), are still there and looking good and displaying the photo.
Look for Facebook here: https://www.facebook.com/StichtingHUVO/
For original message: http://www.huvo-voorschoten.nl/nieuws/
I hope you can give some place to search or if it is a bug, a new version.
I have tried to find a solution in Internet, but not found any.
Maarten van 't Hof