Background Image Help

[Chaos Residue]

If this question has been asked before then I apologize. I have searched Google and this website and found nothing that answers it so far.

I have a background image for my website that is 1680px by 1050px. I want this background image to be a fixed image and I want it to automatically re-size depending on the users screen resolution. Is this possible? If so, what do I need to do to make this happen?

If someone needs an example of what I mean, use this as an example. If someone has their screen resolution set to 800x600 then I want them to see the background image as a 800x600 fixed background image. If they change their resolution to 1280x960 then I want them to see the background image as a 1280x960 fixed background image. If they then change their resolution to 1680x1050 I want them to see the background image as a 1680x1050 fixed background image.

Screen Resolution = 800x600
Image Size becomes 800x600

Screen Resolution = 1280x960
Image Size becomes 1280x960

Screen Resolution = 1680x1050
Image Size becomes 1680x1050