Default Sites vs Site for each domain

I've got an app I'm building. It's a multi domain app for a client. They have different restaurant brands but they want the same website with some custom stuff for each.

Anyways got the whole thing built out and I'm trying to deploy. I am using the domain name to determine what data to display. The problem is that when I set the site as default, it is pulling the domain as 'default'. If I set up a domain or site for each one of the client's domains or brands, when I upload a photo to the admin.domain.com panel then it only saves it in the public directory for admin.domain.com but I need it accessible for all.

How can I handle this and account for it all?