How to keep the body (content) of website alligned with the center of background image

How to keep the body (content) of website alligned with the center of
background image

I'm trying to keep my site's content perfectly aligned with the center of
its background image, is there a way to do this?
Currently my background is
body {
background: url(http://nickhumphries.com/images/tucson-bgl3b2.jpg)
no-repeat center center fixed;
background-color: #fafafa;
-webkit-background-size: 100%;
-moz-background-size: 100%;
-o-background-size: 100%;
background-size: 100%;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
text-align:center;
}
#mainContainer {
margin:0 auto;
}