Go to your Modify Journal page (https://www.insanejournal.com/modify.bml). Scroll down to the bottom and use the override code found below in your "Overrides" box.
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: