Many people don't know that they can't just save any random picture off the Internet and use it. Much of the work online is subjected to regular copyright laws, and many creators aren't fond of people just ripping off their work.
Creative Commons is a new type of intellectual property licensing that allows people to use content in a variety of ways. It's very useful when you're looking for graphics, video, or audio. See the "Licensing" tab of this guide for more info on Creative Commons.
If you're looking for media to use, check out CC Search. It's the Google of Creative Commons works. Just select your media source.