Pictures of American presidents can be found on the website About.com in the section on American history. Portraits, sketches, engravings and photos of all American presidents from George Washington to Barack Obama can be found on the site.
Some of the presidential photos are not in color, since they were created before the development of color photography. The photo of Richard Nixon and those that followed him in office are in color.
Portraits of American presidents can also be found on the American Presidents-Life Portraits website. On the Wikipedia.org website, a list of American presidents, along with a portrait or photo for each, is provided.