How is the News Link Preview Image selected?

Hi,

When we create a news link to an article on a news-based site, the news link correctly pulls in a preview image for the article form the site. However when we create a news link to our own public facing .com site, no preview image is pulled in.

How does the news link code select the preview image to be used?

I suspect there is some tag/code in the HTML of news articles that defines the article image that is missing from the pages on our .com site - can anyone explain in more detail please?

Thanks!