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. [3]
[I would say that since this photo was taken by a US NASA astronaut on a space flight, and then published, then it comes under the category of a US Government photograph in the Public Domain. Makes sense?]