Custom Actions Based on URL

Question

I am developing a portal site that will serve as a resource for multiple companies.  Companies will create their own subdomain and redirect traffic to my shared websites IP address.

Based on the URL of the visitor, I'd like to display a custom image to the user (the logo of the company they are visiting)

How can I easily check the HTTP headers and update my Image control so that I can have logic based on the visitor URL.

Thank you