Yeah, it's actually nice and simple to sort that out.
The file you need to edit is located in /templates/subSilver and is called overall_header.tpl
The part of the code you are looking for is:
<td><a href="{U_INDEX}"><img src="templates/subSilver/images/logo_phpBB.gif" border="0" alt="{L_INDEX}" vspace="1" /></a></td>
<td align="center" width="100%" valign="middle"><span class="maintitle">{SITENAME}</span><br /><span class="gen">{SITE_DESCRIPTION}<br /> </span>
This piece of code is what gives you the standard phpBB logo on the left hand side, and the site name and description in the centre.
Now, depending on how you want it set up, depends on what you want to do next.
This is what I've done. I've removed the site name and description information, and just moved the logo information to the middle.
This is the code we have:
<td align="center" width="100%" valign="middle"><p><a href="{U_INDEX}"><img src="templates/subSilver/images/banner/rotate.php" border="0" alt="{L_INDEX}" vspace="1" /><br>
If you replace the first bit of code with with the second, you get a banner in the middle. You'll need to set the img src to the location of your banner, as I've got a seperate script running which pulls a different image each time the page is reloaded, which is called rotate.php
Hope this helps, and don't forget to back up the original file first, just incase it doesn't go according to plan.