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.
If you want to place a video or image inline with your text just copy the URL while on YouTube etc. then past it in your message as a text only. It will then embed that video into your message as shown here.
if you paste it as a link it will end up like this: Taylor Swift - Lover (Official Music Video) (youtube.com)