Most image sources will include information about the copyright status. Many sources will allow you to reproduce images for teaching or other non-commercial uses. Always remember to attribute the source of the image when incorporating it into your own presentations.
Some image search engines allow you to limit your search to images licensed with Creative Commons licenses that may allow re-use. Often this is on the 'Advanced Search' screen. See the specific license to see if you can modify the image, use it for commercial purposes, or need to make attribution to the creator.
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