I import all my photos to windows photo gallery

1.  All my users have access to all photos no matter what user imported them?

2. They even go to a user  that we never open ( store demo)  I opened it and it had 65GB of pictures on it, does this mean  I have 260GB of pictures? for 4 users?

3.  Is it possible to keep everyones pictures to themselves?

4.  My computer says C Drive No Disc Space, I can't even update virus protection

5.  If I delete pictures from Store demo will it delete them from everybody?

6.  I was going to back them up on portable hard drive but didn't want to have 4 copies

I appreciate any help I can get