It's not something you do from the admin panel.
The easy way to do it is to name the image you want to replace it with:
Then, use your FTP program and upload it to:
Let it replace (Over write) the current logo_phpBB.gif that is in that folder.
That's the way I did mine. It seemed the easiest way
I was puzzled at first though, as there wasn't a file to overwrite. But once uploaded it worked perfectly.
PS: Depending on how you installed phpBB, you may have to get your files transferred from the server to your computer. My install process put them direct to my IP, and did not install them on my computer, so I had fun and games at first workig out what to do, as I had nothing to do it on
Got there in the end though..