Phpbb is one of the best free software you can use to power a forum.
Many new webmaster find it difficult to change/replace the phpbb default logo to their own logo.
To get started with changing the phpbb default logo to your own, create your new logo 149 x 52 dimension (i.e 149 pixel height and 52 pixel weidth), save as site_logo.gif .
Note you must use this exact dimension and save with site_logo.gif for this to work.
Login to your control panel and view your files (can be done through FTP as well).
The folder where the default phpbb logo is located in images inside the prosilver folder..
Here is the correct path to it
Inside the images folder delete sitelogo.gif and replace with your own logo.
Make sure you purge your temple cache in ACP for the change to occur.
Drop your comments below if any.