Before you watermark or resize any photos head over to your WordPress dashboard and go to NextGEN Gallery > Other Options > Image Options, and make sure 'Backup the Original Images' is checked Yes (Default) if you think you will ever need that original photo again. This obviously uses more space but if it's important to keep the originals intact then it's worth it. This is especially true if you are selling hi-res copies of your photos. On the other hand, if you have no intention of selling hi-res images or are just using your website to showcase your photos then you can make sure this option is selected as 'NO' and save considerable space. Of course, always make sure you have backups of all your original photos somewhere safe.
Well, I just wanted to increase the Max File Size to greater than 50MB so I could upload this stupid 18 second video clip. wpForo didn't want to play nice so I wasted like 10 hours on that little episode.