The header contains three columns .. Menu , Forum's name , Logo
If you want to change the Forum's name or delete it or even replace it with a logo .. you have to find in index.template.php :
<td width="40%" align="center" style="background-image: url(', $settings['images_url'], '/xtHEAD.gif);" bgcolor="#83B3E0"><span style="font-family: Georgia, sans-serif; font-size: xx-large;">', $context['forum_name'], '</span></td>
what you need is change , delete , replace the red code only.
Logo will appear on the far right of the header .. and there is already an image which is found in the (xtEnhanced\images) folder under the name 'xtLOGO.gif' .. you cannot see it cause it has the same background colors ..
You need to open that image in your favorite graphic program and add a logo to it , save it then upload it back to your server.