Edit all the overall_header*.tpl and move the logo code into the central TD of the header table.
The above means that this part in the code would look something like this:
<table id="forumtable" cellspacing="0" cellpadding="0">
<!-- IF GT_BLOCK -->
<tr><td width="100%" colspan="3"><!-- BEGIN ghtop_blocks_row -->{ghtop_blocks_row.CMS_BLOCK}<!-- END ghtop_blocks_row --></td></tr>
<!-- ENDIF -->
<tr>
<td width="100%" colspan="3" valign="top">
<div id="top_logo">
<table class="" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr>
<td align="left" height="100%" valign="middle">
<!-- IF GL_BLOCK -->
<!-- BEGIN ghleft_blocks_row -->{ghleft_blocks_row.OUTPUT}<!-- END ghleft_blocks_row -->
<!-- ELSE -->
<!-- ENDIF -->
</td>
<td align="center" valign="middle">
<div id="logo-img"><a href="{FULL_SITE_PATH}{U_PORTAL}" title="{L_HOME}"><img src="{FULL_SITE_PATH}{SITELOGO}" alt="{L_HOME}" title="{L_HOME}" /></a></div>
<!-- IF S_HEADER_BANNER --><center><br />{HEADER_BANNER_CODE}</center><!-- ELSE --> <!-- ENDIF --></td>
<td align="right" valign="middle">
<!-- <div class="sitedes"><h1>{SITENAME}</h1><h2>{SITE_DESCRIPTION}</h2></div> -->
<!-- IF GR_BLOCK -->
<!-- BEGIN ghright_blocks_row -->{ghright_blocks_row.OUTPUT}<!-- END ghright_blocks_row -->
<!-- ELSE -->
<!-- IF S_LOGGED_IN --> <!-- ELSE --> <!-- ENDIF -->
<!-- ENDIF -->
</td>
</tr>
</table>
And because the logo will then be above the menu block buttons, you should probably change style_p_black.css too (because your logo will start overlapping the links):
#top_logo {
border: 0px solid #181818;
background: #000000 url('images/p_black/top_header.png') repeat-x;
height: 150px;
}
Adjust the height until you have the desired effect
(or cut the height of the image used or play with z-index until it looks good)