Scalable background image with constrainend proportions

For my new portfolio I want to have a single-colored image that my site will “hover” over. Since it’s a .gif and a single color I wanted to make the image as large as possible (in the 1600s range) so that people viewing it on a large browser window would see the image without nasty distortion.

The problem is I want the image, when viewed in an 8x6 window to be scaled down to fit that browser, and “dynamically” scale if the user adjusts the size of the browser on a large monitor to say "16x12 and still see the background image the same as it appeared in the smaller size just larger.

I was wondering if anyone knew a good tip or two with how to get this to work with CSS or JS.

Thanks!

Michael Wesley Stratton