Check out the two screen screen shots. I have a tiled background that’s 1000 pixels wide. Looks good in Rapidweaver.
However if I preview in Safari and, for instance, stretch the browser window on a large monitor (I’m testing this), everything shifts and type extends out past the image area.
Set a max content width to 1000px perhaps? that way the content will never go PAST the edge of the top. The reason it is doing this is because your content is set to browser width and is centred, so when you go past 1000 px wide the content still centres on available screen real-estate.
You say that the background is tiled but it is can only be tiled vertically. If it were tiled on the horizontally as well, it could expand to accommodate wider text. You can do this, although it does need some jiggery-pokery to retain asymmetric features such as the red rule and the folded top of the page. But you may not want to do this.