Featured Images are used to represent a content item (gallery, group, page, blog etc) whenever the content item is shared on social media.
If your featured image is not coming up on Facebook you need to purge the Facebook cached data.
Follow these simple steps to purge your Facebook cached data :
1- Copy the URL of the page for which the featured image is not showing up.
2- Open this link in your browser.
3- Paste the URL in the box and click on the Fetch New Scrape Information.
Now share your page on Facebook with the featured image.