Y'know, I just figured out what you meant by "header photo". I mean, it should be obvious but for some reason I was thinking of something else.
The only way I know to add it (which may cost you extra depending on what the circumstantial problems were) is to do it directly from eBay the way you would when creating a new listing there. Of course, you could only do that (I think) if you didn't have any bids.
Anyway, glad it sold!