So basically set the upload file size limit to 8MB or even 10MB.
In the Control Panel > Attachments page, use the default settings for Image Quality.
Most of your user's 6MB images will be compressed and stored to about 500K - 1.5M
I just tried that but looks like the Forum Group Permissions for Max file attachment size
are restricting to the setting there. My board are set at 4M with php.ini max at 8M.
Saw note in CP > Attachment > Image Quality about Forum "The maximum accepted attachment file size (before processing) can be set within the Forum Manager, within the "Details" of each forum." Mine are set at 4M here. NOTE: not within "Details". Should change to "Permissions"