When setting up a PowerPoint presentation think of each individual slide as a separate page in a paper. And just as the last page of a paper is the reference list, the last slide of a PowerPoint presentation should also provide a reference list. The citation format stays the same; it's just a change from paper to presentation slides.
All sources used (text and image) should be included in the reference list at the end of the PowerPoint presentation. If there are a large number of images, you could have one reference list for text sources (articles, books, etc.) and a separate reference list for image sources. Double-check with your professor to see if s/he has a preference.
Creator’s Last name, First name. “Title of the digital image.” Title of the website, First name Last name of any contributors, Version (if applicable), Number (if applicable), Publisher, Publication date, URL. Access Date.
Vasquez, Gary A. "Photograph of Coach K with Team USA." NBC Olympics, USA Today Sports, 5 Aug. 2016, www.nbcolympics.com/news/rio-olympics-coach-ks-toughest-test-or-lasting-legacy. Accessed 24 April 2018.
Author Surname, First Name or Initial. Image Title. Year. Medium. size. Museum or place image is, Location, URL
Chinese Buddhist. Encyclopedia. Vajradhara kagyu. Accessed January 27, 2019. http://chinabuddhismencyclopedia.com/en/index.php?title=File:Vajradhara_kagyu.jpg