Having a selection of photos from your walk helps keep your memories alive, and really shows off the scenery to readers. As soon as you've published your walk (or at any time later), you can add, or change the photos that are attached to your walk.
When posting a new walk, you'll be taken straight to the "manage photos" page for the walk. However - you can get to this for an existing walk via a link on the walk page as long as you're logged in.
On this page you can add new images, remove images, or choose which image is used for the header image of your walk.