Stock images allow you to make visual presentations without creating the images yourself. There are a wide variety of websites providing stock art ranging from expensive to free and high quality to amateur. The biggest advantage of using a paid stock image website are potentially having a larger variety of quality images in one location and images sets such as one model in different outfits or different settings.
Sources:
Wide Nets (potentially mixed bag):
US Government:
Gorgeous and Curated:
Niche:
When considering using an image be sure you know what the copyright status of the image is. The websites listed above primarily feature images released under variants of the Creative Commons license. Many people think the Creative Commons is a blanket permission to use, but in fact there are different restrictions possible under the CC license. Two common ones are Non-Commercial (NC) and No Derivative Works (ND). Images licensed as NC may not be used for business purposes and those under the ND license may only be used in their original form without modifications. For additional information about Creative Commons visit the foundation that established the CC license here. Some works are classified as Public Domain. These are things for which the copyright status is known to have expired due to age or have been created specifically for the Public Domain. An example of the latter are works by United States government employees as part of their official duties, e.g. pictures taken by a Navy Public Affairs Officer. Be aware that an individual or company contracted with the US government may hold copyrights on works they produce. More information about US Government copyright here.
Another term you will run across while searching for images to use is “Royalty-Free”. Images classified as Royalty-Free may be used without paying royalty fees each time it is used. There may be an initial cost to obtain the image, from a dollar up to hundreds or thousands of dollars.
One site I ran across which made me smile is a NASA archive that features pictures throughout the history of NASA and even their predecessor agency NACA.
Great Images in NASA (GRIN)
In closing, have fun creating visual presentations and if you find a photographer you love buy them a cup of coffee.