style="background-repeat: repeat-y; max-width: 1580px"

to the BODY tag of each page. That makes the image repeat vertically but not horizontally. There will be a BGCOLOR=-colored expanse to the right of it.

EDIT: You want the max-width thing too; that stops the Forum in particular from stretching past the size limit, which is gonna look terrible regardless of whether the image repeats horizontally.