User blog comment:Kirkburn/Introducing the Community Page - a hub for community info/@comment-24082086-20160629062553/@comment-3218221-20160629064914

Are you asking how to customize the banner image?

Check out the help pagefor the new feature. It advises you to upload the banner image you want to the wiki and title it ''' Community-Page-Header.jpg. The image should be at least  ''' 1200x250px.

Following those instructions worked for me, at least. See our wiki's community page for an example.