1.The first way is by changing the path of the logo. Upload your logo to templates/subSilver/images and call it logo.gif or something like that. Then, look (near the end of the document) for:
<td><a href="{U_INDEX}"><img src="templates/subSilver/images/logo_phpBB.gif" border="0" alt="{L_INDEX}" vspace="1" /></a></td>
Simply change the bit highlighted in red to the path of your own logo. Easy or what??
The second way to change the logo is even easier. Just overwrite logo_phpbb.gif with your own logo and upload it to templates/subSilver/images and call it logo_phpbb.gif.
3.There are a couple banner management mods, though you can also just add the html to templates/subsilver/overall_header.tpl and overall_footer.tpl