This way of copyrighting photos, documents, music and videos allows the copyright holder to open up restrictions in some ways. For example an artist can copyright a photo to be "share alike" which means other people may use it, but they must also copyright it as "share alike." The following are some places you can find images with various creative commons licenses. Make sure to check the license on the image before you use it.
The public domain contains materials that are no longer under copyright. For example, all materials published on the Internet by governmental agencies are freely available.
You may use these images without restrictions, however you still have to cite them.