Yes, it will work now no matter where the site is installed, without further editing.

Add to the styling so that the background image repeats horizontally (with the edges matching since you have tweaked them in Photoshop), like this:
Code:
    background-image: url(../images/background.jpg);
    background-repeat: repeat-x;
    background-position: top center;
The "top center" will put the first copy in the middle of the page, with its seams still largely hidden by the 900px wide mainWrapper, and the next copies on left and right making a total width of 1950px before another seam shows.