Dumb question: how the heck do I add a “cover photo” on Mastodon?

When I tap the header it just gives me a blank screen:

@Torrleonard is that using an app? Maybe try adding one using a web browser (I use a Metatext for iOS, but pretty sure I added mine via a web browser.
@matteastnz I’m using the app. Guess I’ll try the web. Thx!