This extension for phpBB resizes avatars that exceed allowed dimensions to those maximum settings. You need at least phpBB 3.2.3-RC1 for this extension to work or manually add the following patch to your phpBB 3.2 installation: https://github.com/phpbb/phpbb/pull/5127/files
Copy the extension to phpBB/ext/ger/avatarresize
Go to "ACP" > "Customise" > "Extensions" and enable the "Avatar resize" extension.