When I embed the home page of SO careers on Facebook, it uses Stanford University's logo as a preview picture:

[![enter image description here](https://i.stack.imgur.com/pbbNJ.png)](https://i.stack.imgur.com/pbbNJ.png)

My only explanation is that Facebook's crawler goes check out the site, and is shown a Stanford job due to its geographical location. Because that's the first meaningful image on the page, the crawler goes on to assume that's a representative preview image for the site.

Bottom line, the site should serve a dedicated preview image [that Facebook can understand.](https://developers.facebook.com/docs/sharing/webmasters)