From "14 Amazingly Free Stock Photo Websites" by Casey Ark:
When you do a Google Images search, the resulting photos are not necessarily ones that you're free to immediately use. In most cases, the photos are still covered by photographers' copyrights.
If you're looking for photos to use for a design and want to keep yourself out of copyright trouble, you need to locate websites that explicitly define the copyright license of each image. For all the sites listed above, the license is generally pretty easy to find. There's typically a description of the license on every page or at least a link to a description. Here are two license types you're likely to find on these sites:
Creative Commons zero means that you can use the photos in any way you'd like, without asking permission.
Creative Commons with attribution means that you can use the photo in any way you want, as long as you credit the creator of the photo.
Attribution is simple: If you include a photo on one of a web page, add text that cites the photographer ("Photo by John Smith") and be sure to include a link to his or her site, if there's one.
Be sure to check each website's license page for specific details.
A version of this article first appeared on PlatoWebDesign.com.
Related: The 8 Secret Places to Find the Best Stock Images
My note: Try using the Copyright Advisory Network, maintained by The American Library Associations Office of Technology Policy, for practical apps that can help you determine copyright issues associated with using other people's work.