img.bg {
    height: auto;
    left: 0;
    min-height: 100%;
    min-width: 1024px; /*adjust this to match image width */
    position: fixed;
    top: 0;
    width: 100%;
    z-index: -1;
}

Note that this approach DOESN'T maintain the image's aspect ratio. For a pure CSS technique that does that you need a CSS3 approach.

100% scale background image (doesn\'t preserve aspect ratio)

image, background

CSS