Hello ringring7,
First ad all this is a forum about SME-server a compleet linux based server solution.
The qustion you ask is going about a PHP pakkacke on these server . I now on the web site off phpbb2.com yo can vind more information off the instalation off PHPBB2.
But whith a littele search by your zelf you find the locations off the logo.
Go to your phpbb2 page punt the mous pointer on the logo pucg the riht button an chosse properties
In the screen you get you find the url off the picture go whith a ftp client to that directoie on your server and change the file whith you own logo but keep the filename the same.
Reload you phpbb2 page and youhave your own logo on your page. Same times the old picture is standing in your own caching memory make a few relouds are wait a time and do it again.
Goodlock and regards
John