Your code was correct, it's just internet explorer is retarded and leaves a little space because of your newline after the <img> tag, just move the </td> up to the same line and it resolves this issue
Code:
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en-US">
<head>
<title>TEST</title>
</head>
<body bottommargin="0" topmargin="0" leftmargin="0" rightmargin="0">
<table border="0" cellspacing="0" cellpadding="0">
<tr>
<td>
<img src="boven.gif" border="0" /></td>
</tr>
<tr>
<td>
<img src="onder.gif" border="0" /></td>
</tr>
</table>
</body>
</html>