CSS Guru's: How to center a background image?

Hi all. Want to center my background image so when views with higher res
that 800x600 the image is centered with black background. Here is my css
code:

body {
background-image: url(../assets/bkgrnd_nottransparent800_600.jpg);
background-attachment: fixed;
background-repeat: no-repeat;
background-color: #000000;
}

What I get is a tiled image in higher res.

TIA

Robert