What I know about network rendering:

When rendering an animation, each client renders a single frame at a time.
When rendering a single frame (still image), the image is broken into several small chunks, which are then divided between the clients.

I'm not sure HOW this division is determined...  Whether each client has a fixed number to render, each client recieves new chunks/frames as it finishes its current job, or all clients render in synch, is something that I haven't figured out yet.