I'm helping in the development of a website, if case you want to know which one it is, it's

http://www.virtualaircraftpictures.com/

Anyways, I was thinking about putting a rank on the main page, ranking the people who upload the most pics. The forum is phpBB 3 based. The file upload is done through the built-in file attachment system of the forum, so the statistics of the uploads are available. I just don't know how to do that. Can anybody help me?

Just grab the details from your database using PHP and MySQL querys.

If you know where the "statistics of the uploads" are, you can do it easily.
For example if they are in the database, just look at the name of the database, the table, and the fields, and get statistics with MySQL querys.
If they are in a log as a text file, just do it with some php scripts, to open the file and extract stats from.