ahh, I didn't put it togeather that the two posts in the forum were from you or I would have figured out own my own that you were using phpBB.
As far as changing the banner on your site, if you just want to change the phpbb logo at the top, get your self an FTP client and replace the file domain.com/forum/templates/subSilver/images/logo_phpBB.gif
with a files of the same size that's yours.
If you want to do more than that, it's probably easiest to find a style that you like over at phpbb.com and just adapt it a little bit to suit your needs. Some of them are allready set up for you to just drop a new logo into.
You'll also find directions on how to create your own style there, but it's not the easiest process in the world.
Hope this helps...