Stock photos don't contain specific artworks, people, or places, but can help you tell a story. Using images in your presentation can help to keep your audience more engaged. Corbis and Getty are the two most well known stock photography sites, but you will need to license their images. The two most common types of licenses are royalty free and rights managed. Royalty free in this context does not mean that the image is free to use or in the public domain, but rather it is free of royalties (paying each time you use an image). This chart helps to explain liscense types. However, there are many sites that offer stock photos and clip art for free. Below are several examples.