The NASA website hosts a large number of images from the Soviet/Russian space agency, and other non-American space agencies. These are not necessarily in the public domain.
The SOHO (ESA & NASA) joint project implies that all materials created by its probe are copyrighted and require permission for commercial non-educational use. [4]
From: http://lasp.colorado.edu/sorce/images/SORCE_litho.jpg Credit: either "NASA" [http://eobglossary.gsfc.nasa.gov/Newsroom/NasaNews/2004/2004072217314.html] or "NASA / LASP" [http://www.nasa.gov/centers/goddard/2004solar.html] depending on the page.