There is a great website put up by Robert Nemiroff (MTU) & Jerry Bonnell (UMCP) of NASA called Astronomy Picture of the Day. Everyday a fantastic picture is posted, concerning some sort of visual image from outer space. Sometimes the pictures are even taken from the ground with the naked eye. Other pictures are taken from huge telescopes, while some of the most spectacular images come from the Hubble and other spacecraft.
Images from the NASA can sometimes be used on your blog or website as background graphics or simple images associated with text. Other pictures are copyrighted and one must contact the maker for permission. Since the pictures are all displayed by NASA, I do not see why permission would not be easily granted. Enclosed at the end of this post is a link to the image guidelines. Check out this next photo that was posted by NASA on Halloween 2006.
If you are interested in using NASA images on your web, here are the guidelines.