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You should consider using the CSS property position:fixed.
That, of course, does not work in IE ... but you can always use Dean Edwards' IE7 patch.
Just out of curiosiy -- why would you not want to use CSS? When there's something readily available, and when there's a simple solution ... why not use it?
I'm sure there are scripts out there that can do what you want. In fact, just go to any page hosted by geocities and the like, and you will find one.
Or, write your own ... Use the body onscroll event, and then reposition the image (which will probably be done using CSS properties, btw) :)