Copyright - it is one of those things we really have to teach our students (once we get our heads around it). It's all very well saying you can't but what can you use for free, without restrictions?
US sites - Using flickr
The Smithsonian Institution - Large photos to be used in a number of ways.
The collection: The Smithsonian Institution
George Eastman House - check out the advertising photos from the 40's
The collection: George Eastman House
The Library of Congress
The collection: The Library of Congress
The New York Public Library
The collection: The New York Public Library
NASA - Perfect for the year of astronomy
The collection: The National Aeronautics and Space Administration
The Kids Collection
the Powerhouse Museum
the collection: http://www.flickr.com/photos/powerhouse_museum/
State library of NSW
the collection: http://www.flickr.com/photos/statelibraryofnsw/
That should do you for now.....