The background is left-aligned on the page, but the content is center aligned. The bg needs to be centred, then it'll fit properly.
The problem is that when you run out of image, it cuts to white, and that looks bad. If you want to have the background look right, it'll need to fade out to the background colour of the page (at the moment #F7F7F7, which is whitish), but to blend with a pic like that, the page bg would probably look better dark, maybe even black.
If any admin wants to test, change the css stlye for html to the following
html {
background: url("images/misc/bg.jpg") no-repeat scroll center top #000000;
}
EDIT: I see that's the method that's been used on the main site, black bg and center align the pic
PS, you can test how it looks without making changes to the system by editing the rendered HTML/CSS in your browser by pressing F12 (Firebug plugin needed for Firefox, integrated debuggers in newer IE and Chrome)