While sharing some Blog post or website article link on Facebook, it automatically finds and shows all usable images available on that article page that you can select to post with article as thumbnail. But sometimes, some images are not shown with the feed. You can rasolve this issue very easily.
All you have to do is to provide absolute url for the image that you want to be available for selection while sharing on Facebook. Facebook usually skips relative path images.
Some people have suggested that you could use following link tag in your header or meta tag:
But both of these abve solutions do not work.
Just provide the full url starting with http:// to your image source and that would do the job.
Hope that helps.