You can find openly licensed (creative commons or public domain) images from a variety of sources. These images can be freely and legally used in presentations, reports, papers, and media projects.
Source: Dorothea Lange (American, 1895 - 1965). Abandoned Dust Bowl Home, about 1935–1940, Gelatin silver print 18.9 × 24.4 cm (7 7/16 × 9 5/8 in.), 2000.50.12 The J. Paul Getty Museum, Los Angeles. Getty Open Content Program.
Public Domain refers to those media which are not protected by copyright or for which copyright has expired (think of classic books like Jack London's The Call of the Wild as an example). Project Gutenberg, for example, provides access to many of these titles. Creative Commons works still retain some rights, depending on the type of license used: some may permit redistribution, editing, and even commercial use (the most liberal standard, called a CC BY license) as long as the proper credit is given, while others like CC BY-NC or CC BY-ND may not. Creative Commons standards are available here, and a good summary can also be found in the Harvard Law guide.