OK. So change to setting less than or equal to your php.ini setting for upload_max_filesize
which must be >= than post_max_size integer
. (Reference: PHP.net Reference
My php.ini is set to 8M by default. File sizes larger than that, say 12M, I would need to change both to 12M at least. Trade off would be larger files in storage and slower browser loading speeds.
I dont think you quite understand how any of this works.
The php.net item you referenced is regarding overall uploads that your website can accept, regardless of what you set within your forum software.
The forum software settings will determine what the forum software will allow. This is a custom permission setting per each user group. And the upload will be limited to anything less-than or equal to your php setting.
The image file attachment compression happens immediately after a user uploads their image file.
If A user uploads an 8MB image file, and that file gets compressed to 300KB, the file gets stored as 300KB. It also gets displayed on the web pages as 300KB.
There is nothing larger about it. The original file never gets stored, and because it never gets stored, it is never able to be displayed to any users on your web page. So there is no slower loading of pages or larger storage requirements.