Use clients logo on their login page

Hello, I have Ultimate Branding setup and have my logo on the login page. However, I would like to have my clients logo in their login page, so when their customers and staff go to login to their site, which is part of my multi site network, they see my clients logo and not mine. How do I set this up or add the branding functionality to my sub site and allow for that site to choose its own logo for their login page? Please let me know.

Thanks,

Michael