In order to configure Open Graph tags for Magento 2 product page (e.g.: product image for Facebook) please go to Admin Panel > Catalog > Products and choose a product you are interested in.
Find Open Graph Metadata section.
Open Graph Metadata
1. Set an Open Graph title in OG Title field. In case you haven't set it up the Meta Title will be used if it is configured. Otherwise, the name of the product will be used.
2. In the OG Description set the Open Graph description of the product. Meta description of the product will be used in case an OG description hasn't been set up. If there are none of the descriptions configured, the Default Description will be used.
In order to set the product's Open Graph Image (Facebook image) find Images and Videos section, where you can add a new image or edit an existing one.
Chose the image and set the OG image Role for it.
If you do not set Open Graph Image for a product Default OG Image will be used.
Once you have finished don't forget to press the Save button.