Tracked background does not show in final render. I created a camera tracking animation: a dinosaur walking in my street. Everything worked fine until I deleted the video background and added a new one. In render view I can see the result I want: my 3d dinosaur, the shadow catcher and the video at the background. When I render it, the video does not show. The background is a gray color. 
I don't understand why it doesn't works, because the first video background did render in the final result.

Help!
Thank you! I found the nodes I needed to change just under one button: screen layout 'compositing' > Node tree display and edit 'compositing nodes'. The wrong display was set so I couldn't see the nodes for background.