Anne Blecksmith at CR&L News compiles a listing of seventeen digital image collections available on the Web from universities, public libraries, historical societies, archives, and Library of Congress, including the New York Public Library Digital Gallery with over 600,000 digital images.
To those, I’ll add a couple of places that I visit to find hi-res images:
ARS - Agricultural Research Service image gallery with over 2,000 hi-resolution photos available for download free of charge in subjects such as crops, animals, and insects.
GRIN - Great Images in NASA is a resource with over 1,000 hi-resolution photographic images available free of charge in subjects including space, aeronautics, and Soviet Spacecraft.
NOAA - National Oceanic & Atmospheric Administration’s collection of over 30,000 hi-resolution images in albums such as the Coral Kingdom with underwater images of plants and creatures and the National Sever Storms Lab with tornado and other storm photos.
NPS Digital Image Archives - The National Park Service offers this image resource with hi-resolution photos available free of charge. The archive is searchable by park.
U.S. Fish and Wildlife Service National Image Library - Thousands of free, hi-resolution photographs of wildlife and plants.