These sources will help you find images that are freely available for you to use (so long as you cite them or provide information on their creators). Here is more information on how Creative Commons licensing works, and how to identify items in the Public Domain:
Here is the general APA format to use when citing online images. The rule is to fill out as many of the citation elements shown in the example (while understanding that some of them are difficult or impossible to identify)
Artist or Author. (Year of image creation). Description or title of image [Image format]. Retrieved from http://www.someaddress.com/full/url/
If you are citing a Creative Commons licensed image, a site like Openverse can also provide you with a citation that includes licensing information:
"Cliff Palace, Mesa Verde" by Tjflex2 is licensed under CC BY-NC-ND 2.0.