You are not going to have an easy answer to this. Either you have a table that holds ALL your contents and place the image as the background to that table. But your maximum width on that table can only be the width of your photo. Center that table and use a background color to fill up the empty space.
Even with this option as your page grows longer, your background image will repeat over and over..

BUT, your best best is to use css/xhtml... but you would have to redo the site. With CSS you can set the image to not repeat and not scroll and then you won't have as big an issue.

If you want to cover the width of most sizes you need a minimum of 1500pixels. I've seen some as wide as 2000pixels.