"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 MLA Handbook, images within a research paper "should be labeled Figure (usually abbreviated Fig.), assigned an Arabic numeral, and given a caption. See the example below:
(click on image to enlarge)
Fig. 4. 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. 1924. Schomburg Center for Research in Black Culture / Photographs and Prints Division, New York. Web. New York Public Library Digital Gallery. 1 April 2011.
Google Images is a great tool to see the work of many photographers. For your presentations, however, the images may not be high enough resolution to project well. It's possible to find useful images with introductory information, and you can even use the advanced settings to limit to works that are in the public domain
Wikimedia Commons contains, according to the site, a collection of over 58 million freely usable media files to which anyone can contribute."Files include images, audio, video, animations, maps, 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.