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Wikimedia Commons - site where people can submit media that is available for re-use
morgueFile - site with many photographs, all with a creative commons licence
dreamstime - site for stock photography with a selected number of free photos
Unsplash - site with high resolution photographs that are free to use
pixabay - site with free to use images and videos, licensed under a CC0 license.
Finding photos to use as background images for your slides can be tricky. Especially if you don't want to pay for them. One good option is to search for photos with a Creative Commons license. These are often free to use and re-mix for non-commercial use, as long as attribution is given. The specifics of each creative commons license can vary, so it is important to check the license on each individual item you intend to use.
U.S. Governement photos
Works made by U.S. Government agencies and their employees are, by law, free of copyright. Thus many photographs on government websites such as NASA and the National Parks service, are available to use in presentations. Some agencies request attribution.
If you are willing to be a licensing fee, you can purchase stock photos to use in presentations. As with all photos, be sure to check the terms of the license to see which fees apply to your intended usage of the image.