AFAIK RDP doesn't work like that. It doesn't transfer a bitmap of the whole screen, but instead hooks into the window manager at the GDI command level.
So if a program on the server calls the DrawLine function, the TS redirector captures that function call and its parameters, encodes them into the network stream and sends them to the client. The client decodes the paremeters and calls the same DrawLine function on the client device context to create the image the program on the server is rendering.
Obviously its a lot more complex than that for other cases, and some bitmap images like the desktop background do need to be transferred over the wire, but it uses a tiling mechanism to break the images into small sections, which are also cached on the client so they don't need to be transferred all the time.
Specs of the protocol from which RDP has been extended.
VNC works more like you imagine, it works at the bitmap level, but it uses several techniques to minimize the amount of transfer required. It again tiles the desktop image, so it only need to transfer the contents of tiles that have been changed, compresses the bitmaps, and does not usually include the cursor in the bitmaps, so simply moving the mouse around does not generally require lots of new tiles to be sent unless it changes the state of the image in other ways such as roll-over effects.