Well, I do beg to differ, Sandy. From my experience, this one DOES center reguardless of screen resolution and DOES work with Netscape 6 (notice the check in the code). Click the link and try it out.http://www.dynamicdrive.com/dynamicindex3/fixback.htm
It is relatively simple code (isn't that usually the best) that goes like this: (I'll leave credits in place)
<script language="JavaScript1.2">
/*
Watermark Backgound Image Script- © Dynamic Drive (www.dynamicdrive.com)
For full source code, 100's more DHTML scripts, and TOS,
visit dynamicdrive.com
*/
if (document.all||document.getElementById)
document.body.style.background="url('myImage.jpg') white center no-repeat fixed"
</script>