Add, edit and remove links from your top level navigation - that's the sidebar down the left hand side.
The page descriptions appear in the explore section as well as at the top of each page.
Page titles show in the hero at the top of each page, the page tab, and in the 'explore' sections.
Page images appear at the top of each page and in the explore section.
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.