Changing Header for Multi-Site Users

Hi,
I have a custom made logo that I have set as a default image to show in the header throughout the network of site. When a user signs up for their sub-domain, I'd like my custom logo to show by default as the header, which is does now, but I'd like the users to be able to change the default header and replace it with their own. I've set up some sample sites and when try to switch headers it doesn't show up. Only the default header shows.
How can I correct this?

Thanks,

Jafar