The following sites are contain graphics that may be used without obtaining further permsissions from the author.
Contains 35 million photos. The "embed feature makes it easy, legal and free for anybody to share our images on websites, blogs and social media platforms."
Library of Congress American Memory
"American Memory is a gateway to rich primary source materials relating to the history and culture of the United States. The site offers more than 7 million digital items from more than 100 historical collections." Some materials may be copyrighted and used only under fair use guidelines. The Library of Congress provides some assistance in determining the copyright status of items, and a Copyright, and Other Restrictions page.
National Archives and Records Administration Archival Information Locator (NAIL)
Contains many online photographic exhibits, including Picturing The Century containing photographs from every decade in the last 100 years. Usage guidelines are availabe here.
A gateway to government science information provided by US Government science agencies, has an Image Search link under Special Collections to enable users to quickly find science images, including animal and plant, weather and space, and earth and sun images and more. The information is free and no registration is required.
Includes medical and social history, contemporary healthcare, and biomedical science. Over 100,000 images including manuscripts, paintings, etchings, early photography and advertisements. Released under the Creative Commons Attribution (CC-BY) licence.
U.S. Geological Survey Photographic Library
Over its long history, the U.S. Geological Survey has taken many, many, photographs. In the course of their various geological studies and explorations, persons in their employ have documented volcanic explosions, mining projects, and dams. This website contains 30,000 photographs from 1868 to the present, and many of these images have never been published in any form. New users can get started by clicking one of the subject areas on the left-hand side of the homepage. These areas include "Earthquakes", "Mines, Mills, Quarries", and "Mount St. Helens". Visitors can also perform basic keyword searches, and they might wish to try out words like "dolomite", "karst", or "Colorado". Also, the site has an excellent "Portrait Gallery", which contains images of famed geological pioneers, such as Chares Van Hise.
The following is a list of websites through which you can find and download graphics. Materials located in this manner may or may not be copyright restricted.
Google Image Search
You can search for images just on government websites by adding site:.gov in the search box, e.g. mercury site:.gov
U.S. Government Photos and Images
"Some of these photos are in the public domain or U.S. government works and may be used without permission or fee. However, some images may be protected by license or copyright. You should read the disclaimers on each site before using these images."