When sharing a link in Facebook, it usually should include a thumbnail image of the page, the title of the page and text content below. However sometimes the image is not included so you need to follow the steps below to make the image show up.
1. Edit the page - if you're publishing an untouched page template, the image will not show, there should be some changes on the page.
2. Publish the page
3. Go to this page https://developers.facebook.com/tools/debug/sharing/
4. Paste in your URL to that page to debug
5. Share the URL in your Facebook page