Finding images not under copyright, for presentations:
Limit your search to images that can be reused in various ways with these search tools.
It is easy to find and download images from the Web. However, your use of these digital images downloaded from the Web or scanned from a print source may be restricted by copyright.
The Digital Image Rights Computator can help you figure out whether or not you can legally use an image.
Creative Commons Licenses
Creative Commons is a non-profit organization that has developed a standardized ways for creators to determine which rights they reserve and which ones they waive. Even though you do not need to obtain permission to use these images, they still require attribution (citation) when they are used.
Works by the U.S. Government are considered in the public domain and are not covered by copyright.
Some styles such as APA provide specific citation styles for images from the Web. The CSE (Council of Science Editors) Manual does not have a specific citation format for Web images. Generally, the following information, when available, should be included in your citation:
Information on Citing Images from Wikimedia Commons (from Scientific American)