Place image at top of every site

Is there a way to place a linked image at the top of every sub-site, but preferably not the main site?

All our sub-sites are part of a news network for the city, so I want people to easily be able to get to the main site from any sub-site without messing too much with the design of the sub-sites, so figure an image at the top which couldn't be removed was the best way.