Isolate subdomain in order to add image to header

Hi

I am using multisites and have a number of subdomains based upon country i.e. uk.mysite.com, us.mysite.com, pk.mysite.com

In the theme header I would like to display a flag icon based on the subdomain so that uk.mysite.com would display image uk.png etc

Is there a piece a code that would do this or is there is a better way?

Thanks
Mark