This is based off the work of josegf from this thread.
https://www.phpbb.com/community/viewtopic.php?f=466&t=2285776#p14197016
PHP 5.5+ GD library installed phpbb/phpbb#4500
Copy the extension to phpBB/ext/jmz/avatarresize
Go to "ACP" > "Customise" > "Extensions" and enable the "Avatar Resize Extension" extension.
This if fully dependent on this pull request phpbb/phpbb#4500