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If the image is your own, no need to cite. Otherwise, cite the source! (Notable exception, PowerPoint clip art, but be careful about searching for additional images outside of PPT.
Provide credit by acknowledging who created the image and where you retrieved it from. This can go on the slide where you use it, or on a slide at the very end of a presentation. E.g., AwesomeImage created by SomeoneAwesome, Retrieved from URL.