Set stage background image 100% width / fullscreen

Hi, i am trying to set my stage bg image to 100% width, but im missing the code to do that,

i have in the stage actions

$("#Stage").css("background-image","url(images/bgImage.jpg)")

but with that the image is shown multiple times filling the browser.

how should i set the width property inside that code line?

or how can it be solved?

i had it working before placing the image in the stage and changing in the properties panel to % width and height but is only working in Safari and Chrome, not in firefox and IE  you can see this here:

http://dessiner.mx  (safari and chrome show my img fullscreen everything good, but if you see that in firefox or IE you will see the image in the top left corner)

so that's why i tried to do it in a different way but im missing the width code line for that stage actions panel.

thanks