It's impossible to gauge the bandwidth increase you would experience because so many factors go into that number - the number of images, the size of the images (or videos), the number of people viewing the images (posters is one indicator, but the number of "guests" or viewers is usually much higher).
There really is no way to give you any kind of indication of what you can expect; but you have many controls in our products to help limit or open up your site to less or more traffic (limiting size of uploads, limiting who can see the uploads, etc).
No, you cannot link to content hosted offsite; so that wouldn't work.
Please do not PM me for support or sales questions. Thank you for your understanding.