When I said the word "FREE", what I mean is you do not have to pay a single dime to use these images.
Totally free of charge.
Note: But, I am NOT saying all the images from these sources can be freely, as you like.
You have to check the if there is any restriction mentioned for each image.
Some of these images, you may need to add attribution, or you can only use them for non-commercial purposes only.
The important thing, always check its licensing policy, so that you don't get tangled up with copyright infringement.
Free Images From Museum, Library And Government Websites
Here is a definite list of websites with free images, gathered from museums, libraries and government websites.
Metropolitan Museum
On February 2017, all images of public-domain works in The Met collection are available under Creative Commons Zero (CC0).
It has more than 375,000 images of artworks from our collection to use, share, and remix—without restriction. (Read here for details)
Terms of use.
Rijksmuseum
Any item that can be used for free, you can see it at the item description entitled "Acquisition and rights copyright Public domain".
National Gallery of Art (U.S.)
It has more than 45,000 open access digital images up to 4000 pixels. They are available free of charge for download and use.
You know it is a free image, when it mentioned "Image Use open access" next to the picture.
Terms of use.
The Walter's Art Museum
Terms of use.
Statens Museum For Kunst (National Gallery of Denmark)
Terms of use.
Museum Of New Zealand
Click on the image and you can see the license information below it.
You can also use the filter from this site to search for your free images.
Yale University Art Gallery
Terms of use.
Wellcome Library Images
Terms of use.
NASA: Image Galleries
Terms of use.
Visible Earth
A catalog of NASA images and animations of our home planet
Terms of use.
NASA Technical Reports Server (NTRS)
You can find aerospace-related citations, full-text online documents, images and videos.
The types of information included are: conference papers, journal articles, meeting papers, patents, research reports, images, movies, and technical videos
Terms of use. (Scroll down to read: Photography)
NASA: Gateway to Astronaut Photography of Earth
Terms of use.
Digital Public Library of America
Terms of use.
National Oceanic Atmospheric Administration
Almost all of the images in the collection are in the public domain.
United States Department of Agriculture Image Gallery
Stated on the site: Photos in our Image Gallery are copyright-free, public domain, images unless otherwise indicated. These photos may not be used to infer or imply ARS endorsement of any product, company, or position.
Library of Congress: Civil War Glass Negatives and Related Prints
Terms of use. (Scroll down to read: About Copyright and the Collections)
Library of Congress: Prints & Photographs Online Catalog
Terms of use. (Scroll down to read: About Copyright and the Collections)
Library of Congress: Farm Security Administration/Office of War Information Color Photographs
You can find 1,600 color photographs created by US government photographers working for the Farm Security Administration and Office of War Information from 1935 to 1945.
The photos depict life in the United States, including Puerto Rico and the Virgin Islands.
The pictures focus on rural areas and farm labor, as well as aspects of World War II mobilization, including factories, railroads, aviation training, and women working.
The library does not own the rights to materials in its collections. You should determine for yourself whether or not an item is protected by copyright or in the public domain.
Farm Security Administration/Office of War Information Black & White Photographs
You can also find the original black & white photos here.
Terms of use.
Library of Congress: Daguerreotype Portraits and Views 1839-1864
You can find approximately 700 daguerreotypes in the Prints & Photographs Division.
The majority of the images are portraits, but the collection does include a few early architectural views, outdoor scenes, and copies of works of art.
Terms of use.
Library of Congress: American Memory Collection: Photos and Prints
You can find photos and prints in the American Memory Collection. I have gone through some of the images. I found many of these collections are in the public domain in the US.
But make sure you check the “Copyright and Other Restrictions” page for each collection you’re interested in using.
I found that not all of the content here is in the public domain. Items that are protected by copyright are generally marked as such, but you must check the restrictions page to find out the particulars of each collection.
Terms of use.
The National Archives Catalog (from NARA)
It stored over 57,000 photos.
Terms of use. (Scroll down to read: Copyright, Restrictions, and Permissions Notice)
National Archives and Records Administration: World War II Photos
Terms of use.(Scroll down to read: Copyright, Restrictions, and Permissions Notice)
US Fish and Wildlife Service Digital Media Library: Images
It is a searchable collection of selected images, historical artifacts, audio clips, publications, and video, most of which are in the public domain.
Those not in the public domain are intended for personal, private use.
Terms of use.
U.S. Air Force Photos
Terms of use. (Scroll down to read: Overview)
NorthWestern University: War Posters
There is no terms of use page. But you can see the licensing rights next to the word "Rights" under each poster.
It reads: Materials published by the U.S. Government Printing Office are in the public domain and, as such, not subject to copyright restriction. However, the Library requests users to cite the URL and Northwestern University Library if they wish to reproduce images from its poster database.
The New York Public Library: Digital Collections
Terms of use. (Scroll down to read: FIND ITEMS)
USGS Denver Library Photographic Collection
Its photographic archive collection has more than 500,000 still photographs, slides, glass plate slides, lantern slides, negatives, and sketches dating from the 1870s.
Terms of use. (Scroll down to read: Copyright and Credits)
Naval History and Heritage Command Photo Archive
It’s photo collections are related to the US Navy (or other navies when specifically associated with US involvement or historical events) with specific emphasis on ships, aviation, places, wars/events, activities, and significant individuals associated with naval history.
Stated on the site: Most of the photos found in our collection are in the public domain and may be downloaded and used without permissions or special requirements (those which are not will be noted in the copyright section of the image description)
Terms of use. (Scroll down to read: Copyright and Citation)
Antarctic Photo Library
Here you can find hundreds high and medium resolution photos of wildlife, people, transportation and scenery, from the US Antarctic Program.
Terms of use.
Franklin D. Roosevelt Presidential Library and Museum: Historical Photographs
It has thousands of online images from its large collection of public domain photographs. And also photographs of Franklin and Eleanor Roosevelt, the Great Depression and the New Deal, and World War II.
All are public domain photographs, unless otherwise mentioned in the caption.
U.S. Department of Defense Search: Photos
Terms of use.
Sino-Japanese War of 1894-1895: As Seen In Prints And Archives
Stated on the site: All images of the prints used on this website are provided by the British Library and are in the public domain.
Terms of use. (Scroll down to read: Use of images provided by the British Library)
Europeana
It is an online collection of content from European libraries, archives, museums and other institutions.
You use the filters on the left side of the page to look for your free images.
Scroll down to "Show specific rights statements".
Click on it and it will drop down a list of options to choose from: CCBy, CC By-SA, Public Domain Marked and CC0.
Terms of use.
Folger Shakespeare Library
Terms of use.
United States Department of Agriculture
Terms of use.
Yellowstone's Photo Collection
Terms of use.
You can find more from Wikipedia: Public Domain US Government Resources.