It appears that for a div, IE in standards mode (with a strict or transitional doctype) adds padding and margins to the width.
In quirks mode, without a doctype or with an incomplete doctype, it subtracts them.
Mozilla adds them all the time.
I don't think this is the same for IE5 though. Maybe I'll test it out later.
Try testing this page with and without a doctype:
[HTML]<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Test page</title>
<style type="text/css">
<!--
#first {
border: 1px solid #000;
width: 200px;
}
#second {
border: 1px solid #000;
padding: 50px;
width: 200px;
}
-->
</style>
</head>
<body>
<div id="first"> </div>
<div id="second"> </div>
</body>
</html>[/HTML]
This means that for an image in IE in quirks mode, padding is simply ignored.
Unsolved 
