The United States Geological Survey maintains a wonderful public domain library at this address: https://www.usgs.gov/products/multimedia-gallery/overview (external link). All will do for web work and many are printable quality.
Nearly all 10,000 images are at no charge and copyright free but of course you should credit the photographer and agency whenever possible. I particularly like the well done images of minerals.
Mark Wipfli, Alaska Cooperative Fish and Wildlife Research Unit. Public domain.
David Pyke, USGS . Public domain.
(Credit: Carlin Green, USGS. Public domain.) USGS description: A sample of azurite, the blue mineral, and malachite, the green mineral. Both azurite and malachite are copper minerals that were once used as pigments but are now mostly valued as collectors minerals. They do serve as good indicators of copper deposits that can be developed.
John Mars, U.S. Geological Survey. Public domain. Taking notes on a silicified ridge at Goldfield, Nevada. Silicified ridges are typically associated with epithermal gold deposits.
