(img is about 600px high, it's the only element on the page.. I want it to be centered on the page, both horizontally and vertically.. how do I do this..)
If your image is the only element on the page I would be inclined to use Paul's "Easy Vertical Centering" method.
The updated method uses a float and it is discussed about half way down the page as "The Fix".
You could just set the static #hoz div to the same size as your image and it should work out fine. You could even set your image as a BG image on that div.
The #vertical float div sits on it's own above the #hoz (content) div and controls the vertical centering, view the page source of the link below for details.
Here is the live example of the float method.
Drag your browser window around and you will see that it does just what you are wanting.