Google Images has a built-in filter that helps you find images that are copyright free. See the steps in the graphic on the left for directions. The most commonly used license is the "Labeled for noncommercial reuse" license.
Once you find an image, be sure to click on the image and check for any more specific directions. Public Domain images do not need to be cited. These are the ideal type of image for use in work.
Photos can be used in a design when you wish to convey emotion or add specific details or context. If you looking for a simple image, try adding the word "icon" to your keyword search. If you are trying to find the outline of an image or a completely filled in icon, try adding the term "silhouette" to your search.