img1.png is closest to the viewer and img4.png furthest.
This will create a feeling of stuff moving around when resizing the browserwindow.
body {
background: url("../backgroundForNotSupportedBrowsers.png") repeat-x fixed -80% 0;
background: url("../img1.png) repeat-x fixed -130% 0, url(../img2.png) repeat-x fixed 40% 0, url(../img3.png) repeat-x fixed -80% 0, url(../img4.png") repeat-x fixed 100% 0;
background-color: #123456;
}