Eduardo Macedo on "Open graph and the featured image w/out plugin"

How to set the open graph to catch just the article's featured image?
Example: paste the link http://maisfps.com/heranca-maldita/ on Facebook. I can choose between the articles images (inside of it) and the author avatar, but there's no featured image.

All I've found are plugins to do that and I don't wanna one more plugin.

Someone here know how to set the open graph to show the featured image only?

The third option lists a potential solution (without a plugin) here: http://www.wpbeginner.com/wp-themes/how-to-add-facebook-open-graph-meta-data-in-wordpress-themes/

I could copy the meta code use it inside my header.php?

PS: My default opengraph code is located at header.php.