Many government agencies have online libraries of digitized images and artwork from their collections. Many of the images are in the public domain and not under copyright. Only a few of them are listed here, keep searching for more!
Library of Congress Digital Collections (includes photographs, cartoons, baseball cards, maps, prints, and much more)
National Agricultural Library Digital Collections (The USDA Pomological Watercolor Collection has 7,497 watercolor paintings, 87 line drawings, and 79 wax models of fruits, nuts, and plants)
"Jupiter's Bands of Clouds" "General Store and Post Office" by Doris Lee "Sea turtle"
NASA Image Gallery U.S. General Services Administration Thierry Work, USGS