Page images appear at the top of each page and in the explore section.
Image dimensions: 1920x1080px (width x height)
Find the page you want to edit.
Hit the cog in the top right and select 'edit page'.
Across the top are a number of options, select 'Metadata'.
In the 'image' section at the top, upload an image and add a description.
Scroll to the bottom and hit save.