Prevent Page Background from repeating?
 

So for some reason I've found my page background repeats multiple times on certain pages, for example when viewing a thread the PB repeats itself with every post. It's difficult to describe so I've provided some images:

http://i41.tinypic.com/flwshz.png
http://i42.tinypic.com/ezr42e.png

Also if you visit my site here and browse around you should notice it.

It's not a problem if I'm using a colour for the page background however when using an untiled image it makes the layout look horrendous. Does anybody know how to fix this?