This is a beautiful scene!
I have a few suggestions if you'll have them (I apologize in advance if I wound harsh
-OK! First thing I see when I look at this scene is the background, followed very quickly by the rock on the right, the woman, then the trees. Both the woman and the rock are a llittle dark, but that's OK for now. The background looks great - so, I'll move to the trees. The trees look very sharp compared to the rest of the image, mostly because they have such hard edges. There is actually a very simple fix for this: Using the Select-by-Colour Tool (or if that's unavailable, the Fuzzy/Magic Selection Tool), select the empty pixels behind the trees. Adjust the options for the selection tool to your selection is feathered/blurred by 1 to 3 pixels. Then, simply press Delete. If it works, the edges of the trees should be much softer, but still hard enough they look realistic. Play around until you're happy.
-Second, a biggie... Water is a terrible enemy to photomanipulators because water is a mirror. In your scene, the water is reflecting a sunny blue sky with green trees. Try creating a mirror image of the nebula in the water, then adjust the opacity/transparency so you can see the ripples. If it still doesn't look right, use a feathered selection tool (about 10 pixels this time) to select the existing river and change the Hues so it looks more blue (or if you're using Photoshop, just add more blue to it). This, combined with the mirror-image, should give you a rippling reflection in the water.
-Lastly, the only thing that is missing now is some highlights on the woman. Even in dark scenes, the slightest amount of light will create a highlight. There are lots of ways to do this - you can use a smooth round brush at 5% transparency to draw your own shadows, you can use the Inner Glow Effect on her arms then adjust it so it looks right, or you can even use the freehand selection Tool with a feathered edge to select areas you wish to make lighter, then fill the selected areas with light blue. Best to do this on a seperate layer, then all you have to do is adjust the layer opacity
Anyways, this is a beautiful scene, it just needs a few touchups to make it looks incredible
Once again I apologize if I sounded harsh or offending, it was not my intention
Good luck in the Challenge!