Facebook has recently implemented changes to its systems that have to do with how Shared Links are posted.
When posting a Shared Link to Facebook, Facebook no longer allows services to provide a custom title, body, caption, or thumbnail. Facebook will automatically select the thumbnail based on the OpenGraph tags present in the linked page. If the page has no OpenGraph tags, Facebook will automatically attempt to locate an image to use as the thumbnail.
If you need to modify the thumbnail which is displayed for your site, or for a particular page, you will need to add or modify the OpenGraph data present in that page on your site.
The original Facebook article on the Facebook Developer site can be found here.