At the bottom of the search options, put a check in the box next to "Only search within Creative Commons-licensed content."
When a photographer chooses a Creative Commons license for their work, they are giving permission in advance for others to use their work. Along with granting to permission to use the material, the Creative Commons license may have some restrictions, such as the material may not be used commercially or may not be modified in any way. For more information on Creative Commons licenses, visit:
http://creativecommons.org/licenses/
