Path: Community Settings >> Customization >> Look & Feel >> Preview Image for Facebook Sharing
Administrators might want to show a different image than community logo on social shares like facebook share since they are publicly visible. In such cases this option is very useful.
To add the image simply drag drop it in the below area or you can also choose an already uploaded image from Community file library.
Note: The image you choose will be made public automatically so it's accessible to Facebook Crawler.
Now whenever you want to share an idea on Facebook. Simply click on the 3 dots (more option) and select 'Share on Facebook'.
You will be redirected to the Facebook login page. Fill in your credentials and you will be shows the below page to add details. The 'Preview image' you selected for facebook will appear besides the community name.