Compositor won't load image! The viewer node just shows a black picture. I've checked "Compositing" in render settings. Neither inputting the render result directly nor inputting an external image file seem to work. I'm just learning how to use the compositor and I don't know what to do. Help!

Edit: I'm using Blender 2.76.
To make composite nodes up and running, check two things:


Ensure the Compositing option under Post processing in the Render tab is checked.
Ensure the Compositing nodes are enabled. In the Node Editor, Compositing Nodes tab check Use Nodes: