Google Images can be a great place to go if you want to get some idea of what sorts of images are out there, but it may be difficult to find an image that is big enough and with high enough resolution. It's possible to find that the perfect image, with little or no use restrictions, so give it a try!
Wikimedia Commons contains, according to the site, "a database of 9,907,245 freely usable media files to which anyone can contribute." Files include images, audio, video, and other multimedia. Users can choose from multiple images sizes when downloading, and each image is often accompanied by information about the work depicted as well as copyright information.
Flickr's The Commons: images in the public domain, searchable in one place. Image collections from the Library of Congress, the Smithsonian Institution, The New York Public Library, The UW Digital Collections, and many others.
"The key goals of The Commons on Flickr are to firstly show you hidden treasures in the world's public photography archives, and secondly to show how your input and knowledge can help make these collections even richer."
Just as you would always provide in-text citation information for copied text, you must for images and photographs!
According to the APA Handbook, images, including video, audio recordings, maps and photographs, are cited with creator, their role (e.g. photographer), (year), title, [medium], publisher info.
Fig. 8. (1924). Author Langston Hughes [far left] with [left to right:] Charles S. Johnson; E. Franklin Frazier; Rudolph Fisher and Hubert T. Delaney, on the roof of 580 St. Nicholas Avenue, Harlem, on the occasion of a party in Hughes' honor. [Photograph] Schomburg Center for Research in Black Culture / Photographs and Prints Division, New York. Web. New York Public Library Digital Gallery.
Thanks to Sarah Christensen, Visual Resources Curator at the University of Illinois at Champaign-Urbana and Jesse Hendersen, Visual Resources Curator at Colgate University, for graciously allowing me to copy text and other information from their library web pages.