I first added the HTML strict Doctype because all pages should have one and both IE7 and Firefox displayed the same, which is a good sign.
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
I then added px to the positions and all images went where they should be:-
<style type="text/css">
#layer1 {position:fixed; left:125px;top:10px; z-Index:0}
</style>
<style type="text/css">
#layer2 {position:fixed; left:150px;top:15px; z-Index:0}
</style>
<style type="text/css">
#layer3 {position:fixed; left:350px;top:415px; z-Index:3}
</style>
But you need to check IE6 which doesn't support position: fixed but may use position: absolute instead with the same result (but I haven't tested).
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.