A Facebook share image is an informal way to refer to the image that Facebook will automatically display when you share your website’s content on Facebook in a status update. Here’s an example from PowerUp’s Facebook page:
When we shared the link to that page on our website, Facebook automatically pulled that image. Facebook will try to pull any image it can find on the page to use, but you can specifically set it depending on your website set up. Also, you can set a default image for your entire website, like your company’s logo, depending on your website’s setup.