One of the harder things to do for a presentation is get good royalty free photographs to display in your presentation. Sure it is easy to just google a topic and use the photo that comes up whether it is free to use or not. Sure there are ways to search on Google for Creative Commons images you can use. But you might still not find what you are looking for. The Stocks comes in to save the day. Advertised as "the best royalty free stock photos in one place", The Stocks features many sites that do offer high-resolution photos free for you to use.
As always I offer a word of caution before using these type of sites with students as the pictures do update. So just because you didn't find any inappropriate pictures today doesn't mean there might not be some tomorrow.
Give The Stocks a try.