Background insertion does not work I want to insert an image in the background of my scene. The classic method to do that is with this compositing node set-up:



However, the image node (left of screen) doesn't seem to pick up the background JPG file I assigned to it. I can tell because otherwise, that JPG picture would be showing in that node. And indeed, when I press F12, the picture is not in the background. I know, the Render Layers node isn't showing the scene either, but at least it did its job; the object of my scene appears with an alpha background.

Interestingly, when I start a completely new file (with the Cube), and try the same set-up (with the same JPG file), it does work; the Cube is set against the background of the picture. And yes, the picture shows up in the image node as well. Here's the screen shot:



How come the node picks up the same JPG picture in one Blender file, and doesn't in another?
This happens when you don't check ✓ Use Nodes in the Compositor.
In such case, only the Render Layer will be composited, as you described.