I found an image on the Corbis images website:
(Full image here.)
At the end of the description there is a small note:
This is a publicly distributed U.S. government created image. Image is in the public domain.
If I cannot obtain a copy freely via Corbis (it does not seem that I can), presumably there is a copy somewhere else that is. Would such a source be online, or is the value Corbis adds to the image the fact that it is available electronically?