If you want to include great photos or images in your presentations, here are a few websites to get you started. All images on these websites are under either Creative Commons (CC) licensing or public domain. As in anything, remember to cite your images. For CC, see the infographic below.
Pixabay (photos, illustrations, vectors)
Science Image (photos, videos)
Freepik (photos, icons, vectors)
Wikimedia Commons (photos, videos)
Flickr – search by license