How to enable avatars in phpBB3
Avatars are images with which registered users can customize their account. They are shown under the user's name when the user posts a comment in a forum on your board.
In phpBB3 avatars are disabled by default. To enable avatars and change avatar settings, log in to your administration control panel, then on the General page click on the Avatar settings link on the left. To enable avatars just mark the Yes button for Enable avatars. You also have to specify the way that users can upload or link to avatars. For example, you can mark the button Yes for Enable avatar uploading, and registered users will be able to upload their avatars from their local computer to a folder on your hosting account. The folder is specified underneath. By default it's /images/avatars/upload. This means that, for example, if your phpBB3 board is installed in a folder called phpbb3 in the root public_html directory on your account, the full path would be public_html/phpbb3/images/avatars/upload. If you wish to, you can change the folder in which avatars are uploaded.
After you enable avatars and avatar uploading, registered users will be able to upload and edit their avatar from their user control panel: Profile tab>Edit avatar.
From the Avatar settings page of the administration control panel you can also configure other settings, such as the allowed maximum file size of the avatar and the minimum and maximum dimensions of the avatar image.