Thank you for posting and I'm sorry you are experiencing this issue.
I took a look at your facebook page and it appears when your post is being shared directly from facebook by your contacts it does work properly. What I did notice is that when your contacts are using the share links within the webpage version of the campaign the image does not automatically populate.
This is an issue that we've recently discovered and that our product team is working on resolving. I have created a case within your account and submitted it to our product team. Once there's an update or the issue is resolved you will receive an email communication.