change bg color to image??

Hi all
First of all tnxs for the nice guestbook.
Now my question, i would like if possible to change background with an image?
templates/header.php
body { background-image: url(http://domain.com/xx.gif); }

BUT WHERE change this??? i will have image not color.

Table Background Color

First Alternating Table Column Color

Second Alternating Table Column Color

try body.tpl

body.tpl where is body.tpl

Sorry I meant body.php in templates folder.