[markbnine]

For NASA image data, you can go to the PDS: http://pds-imaging.jpl.nasa.gov/index.html The Atlas has access to the mission released data. There is also link to PILOT, which has quick access to MESSENGER data and some Dawn data. New Horizons will be added soon.

Although there is a learning curve to processing and projecting raw imagery (using ISIS), it's gotten simpler. You can now use gdal to generate images.