Windows has a Messenger service that is often turned off for security purposes. You can turn this service on and use it within your network, but be wary of outside abuse. However, this service was only good for text.
Yes, it is more of a text medium. I was looking around for native Windows solutions to this and Messenger got stuck in head. Before the spammers got their hands on that protocol it was a great way to broadcast messages to everyone in the network.
Windows Messenger looks like a better solution. It's similar to the Instant Messenger (IM) services like MSN Messenger if you needed the images. You would need some further infrastructure in your network like Live Communications Server 2005, Microsoft Office Communicator 2005 or Exchange 2000 Server Instant Messenger Service.