The problem is that it seems like the content your posting is restricted to only authenticated users.
I went to your Facebook page. All of the images have links that are like:
/25-category-in-en/uncategorised/28-indulgence
However, if I click that link, I'm redirected to your home page. The home page has the default image setup for it. Facebook would be redirected to that page as well and detect that default image, which is why that's what it's choosing to use.
If the links are working for you and bringing you to the right article, please log out of your site on the front end and try again. Then, determine why the links redirect you to the home page and correct that.
I hope that helps, but if you need anything else, please let me know.
Thanks,
Alex