From the Research Desk

 

Including a few images can make a presentation much more interesting.  Several sites come to mind:

 

The Library of Congress collections are a good source of such material–photographs, and posters such as the one above:

http://memory.loc.gov/ammem/index.html

 

For scientific subjects one can go to: http://www.photolib.noaa.gov/

 

Note: you should always acknowledge the source of the material even if it is free of copyright restrictions.