You are using an incorrect path.
Don't forget to make sure you copy the image itself to your solution's folder
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>BELLCONSULTS</title>
<style type="text/css">
.mn{text-decoration:none}
#hd{"background-image:url(header.jpg);}
</style>
</head>
<body>
<form id="form1" runat="server">
<div>
<table style="width:1200px; margin-left:100px; border-style:none">
<tr>
<td id="hd" colspan="3" style="height:110px">
</td>
</tr>
<tr>