Can give opportunity for my users to share images to facebook if all pages of the site are available only to authorized users?

We develop educational website for children. All pages are accessible only by password. The site generates images with congratulations on the achieve goals. We want to make it possible to share these images from site.

The problem I see is that facebook cannot access the pages. Which Permissions and Features should I get to give users share images to their facebook profiles from our site? Thank you!

Asked about 4 months ago