this is not a solution to the problem! users complain that they cannot upload images of different sizes. And just leave the forum! WpForo Advanced Attachments would automatically compress images to a preset size that can be customized!
the addon works based on your server power. If your server cannot perform such a big file resizing, then wpForo cannot do that as well.
Your users should understand that the WEB is not a printer and the size of images should be for web not for banner printing. And you should know that the image resizing requires tens of times more server memory than the image size so you should blame your server not wpForo. If one image is uploaded correctly it means the functions of wpForo works fine. And the image specific issues are server specific issues not the addon specific issue.
And, please don't focus on that number. You can set it even 10GB, but your server has limits. Your hosting service doesn't allow you to use that size. Hosting services has hard memory limits and that number will not help you. I don't think your hosting will allow you to use 550MB for each concurrent page call (you may have other page calls at the same time). Here is a good article abut that: https://www.drupal.org/hosting-support/2009-04-17/php-memory-limit-becoming-serious-issue-on-shared-hosts
In any case, I've already mentioned above that there is no way to manage so large images, because it's mostly depends on the server and hosting limits.