Copyright law exists to protect the rights of content creators. However, that does not mean you absolutely cannot use anything that has been copyrighted. Fair Use (Title 17, Section 107 of US Code) allows the use of copyrighted works for purposes such as:
Check out the following video for a quick description of fair use.
The following four factors are used in determining fair use.
Source: Association of Research Libraries http://www.arl.org/publications-resources/3537-fair-use-fundamentals-infographic#.WvRICKQvyUk
Use the following tools to help determine whether or not what you are using falls under fair use.