Google updated image SEO best practices documentation to shed more light on how image thumbnails in Search/Discover are selected (I added the all caps):

> *What*: Added a new section on specifying a preferred image with metadata to the image SEO best practices and Discover documentation.

> *Why*: Based on feedback, we're clarifying that Google uses BOTH schema.org markup AND the `og:image` `meta` tag as sources when determining image thumbnails in Google Search and Discover.