Why Buy The Cow?
The government of the United States of America has a wealth of images, including photographs, maps, flags, line drawings and paintings. They are all public domain. Some images should be credited.
NASA is a great source for some very evocative images. CIA fact sheets are a great source of information.
I should add that you are not allowed free use of photographs showing the faces of government employees.
Credit: U. S. Fish and Wildlife Service
