Add Open Graph Metatag in backend
Provided you installed the Metatag and Metatag Open Graph extensions, you need to:


Go to the metatag configuration page (/admin/config/search/metatag).
Edit the desired config
Under the Open Graph tab, fill the Image field with the relative path to your image

Save your config.
When expanding the config you modified, you can see the list of metatags you set (on my example I edited the Global configuration). You can see that og_image is in it.