Page images appear at the top of each page and in the explore section.
What size image do I need?
620x465px. Anything larger than this will be cropped to fit the space, so text and imagery may be lost.
Editing the image
Go to page
Find the page you want to edit.
Edit the page
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.