Backdrop not available in Nodes In Node Editor, I have selected Compositing, Use Nodes, and Backdrop. In the tutorial I'm using, the object image shows up on the Render Layers Node, but it doesn't on mine and I can't get the backdrop. Can someone tell me what I might have done wrong? This is v2.76, Blender Render.

Edit: Here is the tutorial, Simple Energy Effect. Node work begins at 9:25.
https://www.youtube.com/watch?v=bnrsoSagIU8

As soon as he clicks 'Use Nodes', the left box has the image that will be on the backdrop when he clicks that button. My Node box has no image in it.
The small preview displayed in the render layers node is only visible after you have first rendered your scene. The preview can also be hidden by clicking the little material circle at the top of the nodes.



The image displayed as the backdrop in the node editor comes from the viewer node. This is also not available until the scene has first been rendered.