I'm assuming that's a standard Joomla article. If that's the case, all you should need to do is enable the Open Graph - Content plugin in the plugin manager.
If it's not a Joomla article, let us know what it is. We may have a plugin for whatever extension it is. If not, you may need to add the Open Graph image tag manually by inserting the following 'tag' in the article contents:
{SCOpenGraph image=http://site.com/link-to-image.jpg}
I hope that helps, but if you need anything else, just let us know.
Thanks,
Alex