add this css to your background property:
no-repeat

That will keep it from repeating. If you want to position it in a certain place  you can add top, left, right and bottom to that.

example:
body {
background: url('image.jpg') top left no-repeat;
}

You can also 'stick' the background so that it doesn't move when the page is scrolled with this css:

background-attachment: fixed;

After that, if you still want to position it further you can add margins like this:
margin: 10px auto 50px auto;

The first number is how far from the top of the page you want it, the second value is the right side, bottom and then left side (it goes clockwise).  Auto means that the image will center on the left and right sides but you can assign a number value if you want it in a certain place.


@bejewels - you need to put it in the body portion of a custom stylesheet. the templates don't have a spot for this.