Copyright information from http://visibleearth.nasa.gov/useterms.php - With the exception of images produced by the SeaWiFS, QuickBird, and IKONOS instruments all images on the Visible Earth are governed by NASA's Terms of Use below. [...] For all non-private uses, NASA's Terms Of Use are as follows: 1. The imagery is free of licensing fees 2. NASA requires that they be provided a credit as the owners of the imagery [...]
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. [2]