PHP Code:
<table width="100%" border="0" align="center" cellpadding="0" cellspacing="0" class="header" background="includes/templates/blue/images/hdr_bg.gif">
<tr>
<td width="39%" rowspan="2">
<table align="left" border="0" cellpadding="0" cellspacing="0" width="398">
<tr>
<td width="22"><table align="left" border="0" cellpadding="0" cellspacing="0" width="22">
</td>
</tr>
</table>
<table border="0" width="251" cellpadding="0" cellspacing="0">
<tr>
<td width="251" valign="top" align="left"><a href="./"><img src="includes/templates/blue/images/sitelogo.gif" alt="Home Page" border="0"></a></td>
<td width="500" valign="top" align="center"><img src="includes/templates/blue/images/empty.gif" width="500" height="1"></td>
<td width="251" valign="top" align="right"><img src="includes/templates/blue/images/hdr_mid_pics.gif" width="280" height="80" border="0" alt=""></td>
</tr>
</table>
</td>
</tr>
</table>
Under your includes/templates/blue/images folder, create an image file called: empty.gif and (white color - since your background header is white) and put a width size of: 500px and a height size of: 1px. This should do the trick. 
Of course, do a backup of the header file before making these replacements first.
As for the latest
PHP Code:
</td></tr></table>
HTML tags, I'm already assuming that, based from your post, missing HTML table structures are missing but I could be wrong since you could of also posted a peace of the table only. If so, you'd simply need to make the rest of the arrangements in order to fit right into your site header.