Evidently, phpbb (both 3.1.x and 3.2.x) are using an old version (1.3.4) of Plupload. The old version has a horrible algorithm for downsizing images that are uploaded as attachments. The newer version was released just about exactly one year ago, and remains their current stable release. That new version (Plupload v2.3.1) has a much improved image downsizing algorithm - desperately needed by phpbb for attachments.

Please see this topic (in extension requests {https://www.phpbb.com/community/viewtopic.php?f=496&t=2431441&p=14812611#p14812611}, though this is no longer an extension request) as to why this is an extremely important upgrade of one of the components of phpbb, and I ask for this "improvement"/upgrade to be listed as Major and that it receive high priority to be implemented immediately. Note that if there is backward compatibility, (and I would assume there is), the new plupload should be a drop-in replacement (and if any new features/functions have been added, those can be explored further in other pbpbb version updates at a later time.)