VS 2008 Remote Desktop: Detect which parts of the screen change.

I'm making a remote desktop application which will obviously require a client to take screenshots and send them over to the server which will display them. I have this working except it's extremely slow because it captures the full screen instead of only sending the parts that change. I'd assume that it will just compare two byte arrays which hold the image and send over the parts that change, but I'm not sure.

So basically what I'm looking for is a method to detect where the screen changes and send the portion of the image that has changed over to the server.

I will post the code if needed.