Here is a sample of CSS code that I use on a site to "freeze" a background gif at a specific location on the page. The gif is about 20 percent opaque, so it is really faint on the screen and it doesn't move when the page scrolls.
body {
background-color: #dfffdf; // set the colour for the page
background-image: url('images/opac_logo.gif'); // select the image
background-repeat: no-repeat; // no repetition of the image
background-attachment: fixed; // fix the image on the page
background-position: 50% 75%; // at this spot, play with it...
}
When I view your page, the image was repeated and didn't "freeze" itself on the page. If you want that to happen, feel free to borrow from the code I have posted.