Go to your Modify Journal page (http://www.insanejournal.com/modify.bml). Scroll down to the bottom and use the override code found below in your "Overrides" box.
Note that if you are using more than one GLOBAL_HEAD override (such as a font override and a background override together), you will need to merge them together (http://www.insanejournal.com/community/howto/1020.html).
On some newer browsers, background images can be "fixed," so that the image will not move as you scroll down the page. To do this, just add "; background-attachment:fixed" to the middle line of the above override code, like this: