A Custom Thumbnail is an image that represents a Page both in your Site Menu and when sharing your Page on social media or in a messaging app.
To add a Custom Thumbnail to your Page, open the Page in the Designer.
Click the floating Edit button to open Page Details
Scroll down under Details and you’ll find Description & Thumbnail.
Upload a Thumbnail in the image well.
Publish or Re-publish your Site, then check your Site Menu or share your Page to see your Thumbnail in action!