How does Facebook choose photos from a site?

Question

Facebook

Facebook, at the time of linking another site to its own, not yet completed, post substitutes a photo from the page of this link. It often happens that he chooses any other photo from that site, and not from a specific page (the link to which you put). And so, attention a question: what??? Why can't he calmly substitute a photo from the desired page, and not take it from somewhere else ???