Do you really want your page to work in quirks mode? Your HTML centers fine once I added doctype to to force standards mode:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<title>Welcome</title>
<style>
#pageContainer {width:300px;margin:0 auto;text-align:center;}
#toLogo{border:none; }
</style>
</head>
<body>
<div id="pageContainer">
<a href="http://portal.thesit.com" id="toSite">
<img src="http://stackoverflow.com/content/img/so/logo.png" id="toLogo"></a> </div>
</body>
</html>