##
##        Mod title:  Automatically resize uploaded avatars
##
##      Mod version:  1.0
##   Works on PunBB:  1.2.*
##     Release date:  2007-08-23
##           Author:  thame^
##
##      Description:  PunBB's default behaviour is to reject any uploaded avatar
##                    image whose dimensions exceed those set in the options.
##                    Using this mod, PunBB will attempt to automatically resize
##                    those images to the maximum allowed dimensions, maintaining
##                    their aspect ratio. If the file size of the _resized image_
##                    is larger than allowed by the options, the upload will still
##                    be rejected.
##
##   Affected files:  profile.php
##
##       Affects DB:  No
##
##            Notes:  This mod requires the GD image library, including support
##                    for at least JPEG (GIF and PNG are also recommended). If
##                    your copy of PHP is compiled without GD support (or lacks
##                    support for the uploaded image type), the mod will silently
##                    revert to the default PunBB behaviour. Visit
##                    http://php.net/gd for more information about the GD image
##                    library.
##
##       DISCLAIMER:  Please note that "mods" are not officially supported by
##                    PunBB. Installation of this modification is done at your
##                    own risk. Backup your forum database and any and all
##                    applicable files before proceeding.
##

Download from punres

As mentioned in these threads.  Special thanks to Koos for helping me turn this into a proper mod.