Creative Commons licensing is at the heart of the OER movement. CC allows creators to specify more flexible forms of copyright that allows "others to copy, distribute, and make some uses of their work." There are a range of options for the type of use that CC licenses allow:
Images that are in the public domain do not need to be credited, though you may want to credit the image to help students understand its relevance. Most images created before 1924 are in the public domain. In addition, many images created by government bodies are also public domain. Most Creative Commons images do need to be credited. These credits can appear anywhere in your project, including a closing credits slide, unless an author asks you to credit their work on the same page that you use it. If you want to make things easy for yourself, just keep a list of credits as you go, and list them at the end. A good credit format is this: “Name of item” by [Author]. [Abbreviation of specific license (i.e. CC-By-SA) with link to license] [link to the item]. Sometimes authors don’t provide their real name. In this case “Wikipedia user [username]” is fine.
Best Practices for attributions - You can use CC-licensed materials as long as you follow the license conditions. One condition of all CC licenses is attribution. Here are some good (and not so good) examples of attribution. Note: If you want to learn how to mark your own material with a CC license go here.