Free Images on the Web: Photos and Paintings

General Collections:

Library of Congress American Memory 
CalPhotos has been on the web since 1995 and was one of the first online image databases specializing in natural history subjects.

Online Archive of California (OAC) A core component of the California Digital Library, the OAC is a digital information resource that facilitates and provides access to materials such as manuscripts, photographs, and works of art held in libraries, museums, archives, and other institutions across California. The OAC is available to a broad spectrum of users -students, teachers, and researchers of all levels. Through the OAC, all have access to information previously available only to scholars who traveled to collection sites.
The OAC includes a single, searchable database of "finding aids" to primary sources and their digital facsimiles. Primary sources include letters, diaries, manuscripts, legal and financial records, photographs and other pictorial items, maps, architectural and engineering records, artwork, scientific logbooks, electronic records, sound recordings, oral histories artifacts and ephemera.

PicturesNOW  "A Visual Encyclopedia of FREE still and Video Images.Search and Browse through one of the Webs largest free collections of Images on nearly every popular subject."

ImageBase "The ImageBase is a searchable image and text database of objects from the collections of the Fine Arts Museums of San Francisco (the de Young Museum and the Legion of Honor). The collections (110,000+ objects)..." It is part of the Fine Arts Museums of San Francisco. According to the site it is "...made up of 132549 images with 183 sections organized into 3640 categories. is the largest collection of free photographs on the Internet. (link back and attribution required)...."
My impression is that it is probably not so useful for educators. Photos of nature; transport (e.g) aircraft, airlines, rr); the UK; food and drink; Europe (9 countries); USA (10 states), ect. Site says more than 2,500 stock photos for free. Limited.

Free Stock 
This offers photos for free use, with only a few small restrictions.
The site offers beautiful photos of these subjects: Animals,Christian, Egypt, Israel, Near East, Rome, Scenery, Sky, Weather, Wildflowers, Plants. 

University of Washington Libraries Digital Collections "a growing number of collections from the University of Washington Libraries and faculty in such diverse areas as the humanities, the natural sciences, and the regional cultures of the Pacific Northwest and Alaska. The site includes the award-winning project American Indians of the Pacific Northwest.These collections are especially strong in Seattle history, architecture, labor and industry, the Klondike Gold Rush, and our natural environment. (italics are mine) Materials are primarily pictorial, although some have accompanying essays and texts. Other media are presented, such as newspapers, reports, pamphlets, posters, and maps. More formats will be added in the future. The emphasis of these collections is on rare and unique materials."

New York Public Library Digital Collections

US Government Agencies

Wikipedia Link of Govt Sources 
A collection of US Government Science and Technology web sites

NARA (National Archives and Records Administration) Take a look at the search tool for the National Archives "Digital Classroom," for materials that can be used in the classroom. My impression is that this could be a great source for US history and for anthropology courses with its historical stock footage. Search the word "Custer" and see the classroom materials that you pull up. See also their audio-visual page. You can also go to their homepage to start your search. Try searching "Custer Photographs" and check look for "American West Photographs" in your search results.

NASA Astronomy, Earth, Flight Vehicles and more. 

NOAA: (National Oceanic & Atmospheric Administration ) photolibrary: 
"...the NOAA collection includes thousands of weather and space images, hundreds of images of our shores and coastal seas, and thousands of marine species images ranging from the great whales to the most minute plankton."

NPS: (The National Park Service)  The park's Digital Photo Archive is hosted on Flickr 
Smithsonian Photo and Image Collections

USDA: (United States Department of Agriculture)

USDoD: (US Department of Defense) Images/multimedia, photo-gallery

USFWS: (US Fish and Wildlife Service) Images

USGS: (U.S. Geological Survey) USGS Image Collections 

US Department of the Interior Photo Galleries
Includes: USFWS, USGS, Bureau of Reclamation, NPS
Instragram Search

A list of Federal agencies you may want to check:

Is there a Federal agency related to your discipline? Most of the work created by the US government is in the public domain, so look for federal agencies related to your subject for potential free materials. 

You are responsible for obeying copyright laws and any conditions stated on the sites that you access.