When using Google Documents if you click on the blue share button you can change the sharing permissions. The symbol on the blue share button gives a clue to the sharing status. By default documents are private. Within the domain you may see a picture of a building on the blue share button indicating that only people at your school can view the document. Sharing explicitly with collaborators changes the icon from a padlock to silhouettes of people. A chain link appears below the silhouettes of people if the document can be viewed by anyone with the link. A globe appears on the blue share button if the document is publicly available.
Check out Sylvia
Note: Sylvia allows her images to be used by teachers and posted. Make sure you give her credit and a shout out if you use her work 🙂