Thanks for the feedback guys.
With the moon I just applied the moon imagemap as is, assuming i'd see the "front" part of the moon. I'll look into rotating it.
Badger, Eoin, I applied some very large power fractals to the moon and I wonder if I may have gone too large resulting in the polygonal shape you mention Eoin.
Kadri, what better compliment.

Superza, the moon and stars are actually just image maps and in fact I did use a deafult shader in the background. My current process involves saving the image out of Terragen in exr format and the modifying the levels, brightness, contrast etc in Blender's node editor. The point I was trying to make was that when I currently apply an effect it changes the whole image, sky,water,terrain etc. With the render elements feature i'm hoping I can restrict certain changes to certain parts of the image before combing them back at the end. This would give me much more control over what part of the image got affected when applying effects.