If there is a little icon of the world next to the post or picture on your wall your friends can share it and anyone can see it, if there is an icon of two people next to the post or picture no one can share it but you and only your friends can see it.
If you go into privacy settings you can set limit past posts to friends only, however, profile pictures and cover photos and their comments will always remain public and can be shared.
Any already shared post by your friends will remain shared even if you subsequently set it to private.
Mine is set up so only friends can see my pictures and no one can share any post apart from my profile and cover photos.