You would want to instead put the image in #page in the CSS using the following CSS. Note that the length of the main page might vary in length a little from time to time, so the right length for an image isn’t an absolute. You can have a shorter image and then have it repeat in the “Y” direction (down). If you want the image to repeat down, then replace “no-repeat” in the background declaration with “repeat-y”.
Create your image at 940px wide and whatever length you choose and optimize it heavily so that it doesn’t go over 250k in file size if at all possible to keep page loading times reasonable. Upload that image to your media library, get the URL and replace URL OF IMAGE between the double quote marks in the rule below.