Drupal - Drupal 8 Metatag and Media Entity modules to set meta image tags

If I understand your question correctly you'll need to do the following:

• Check on your modules page that the "Metatag: Open Graph" submodule is enabled.
• Go to metatag config page (Configuration > Search and metadata > Metatag).
• Add a new "default metatag" and select your content type in the "Type" selectbox.
• Fill in the rest of the fields similar like following picture (just don't forget to use your own field names:

Here are some things you need to know about my tokens:

• field_teaser_img: is the fieldname of my media entityreference on my content type.
• field_me_img: is the fieldname of my image field on my media entity type.
• facebook: is the imagestyle name I created for these metatags
• height, mimetype & width: are property fields of the media entity type yet I didn't create those fields but it does seem to work.

I hope this helps.