When editing a page or a section, you will see an option for Preview Photo. This is the photo shown when the pages are listed.
On pages, the first photo uploaded to the page will be the preview
photo. You select a different photo from the gallery by clicking the
photo icon under the thumbnail you want to use.
You can also upload a completely different photo by clicking the Preview Photo tab.
The preview photo is also used if someone likes or shares a page.