Scaling header images for different screen sizes

Hello Chaps

It's been ages since I had to ask something, and it's bound to be easy but I just can't figure this out.

I'm trying to set up a rotating header on http://tinyurl.com/gw-dur. I have formatted two images so that the files are 1920px by 428px. I read your recommendation that the image should be 1920px if that's the possible size of the display used to view the site.

I want the whole image to appear. I've set the header image depth to 428px.

When I view the page it is displaying the image full height, but not scaling down the width to suit the window.

What's the trick to get it to display properly?

Thanks
Graham