Images are generally copyrighted. This means that, in order to use them, you should be getting the permission of the creator of the material. However, there are some sources for copyright-free or public domain images, such as Britannica Image Quest
You can also use Creative Commons (CC) images, which the creator gives you the right to use, just make sure to attribute them properly (see box below).
There are lots of websites that only use CC licensed or public domain images, such as Wikipedia and Flickr. Bing and Google searches also have the option to only show you CC licensed images in their image search, or you can use the CC Search Commons.
When using Creative Commons images in your paper, you should caption the photo with the following:
"The name of the photo" by The Creator of the Photo, licensed under CC by Version
Here's a website I used to learn how to cite images.
My citation for the website would look like this:
Best practices for attribution. (2014, March 5). Retrieved March 30, 2015, from https://wiki.creativecommons.org/Best_practices_for_attribution