How to add background image to Stage?

I use the last version(2014.1.1)

I want add a pic to be a background image in stage.

So I do:

1. Add "compositionReady" in Stage

2. Key-in: sym.$("Stage").css("background-image","url(images/bg.png)");

but I can't see the pic, and in develope tools, the tag didn't have background-image.

How do I solve the problem?