Resize background image to page NOT window

[Kevin]

Hi all,

I have create a background image that fits to the page by using this
code.
<img src="url/bg.jpg"
style="position:absolute;height:100%;width:100%;z-index:-500;">
Now this code creates an image that fits the screen but not the page.
Is there any way (javascript or css) to fit the image to the screen or
to fit the image to the screen resolution but as a fixed image (so
that if you scroll the image doesn't scroll).

Thx,
Kevin