Once you’ve clicked on edit appearance, click on the icon that shows up below the save button. That’ll bring up the menu to alter the header image.

cowscratch:

Nothing shows up… T__T
Maybe Adblock or something is screwing with it? :s

I’ve never had problems with Adblock for mine.
It should be like this.

(sorry, I’m on my mobile, so it makes it a bit harder to show you properly)