NASA are extremely liberal with how you're allowed to use these images. There are other people selling prints of them, for example.
"Unless otherwise noted, images and video on JPL public web sites (public sites ending with a jpl.nasa.gov address) may be used for any purpose without prior permission, subject to the special cases noted below."
The "special cases" mostly deal with using the NASA or JPL logos, or using photos with real humans in them commercially.
http://www.jpl.nasa.gov/visions-of-the-future/images/superea...
I hope the author has permission to use it?