Really nice image, and an excellent color palette you have created with this. I think mainly this critique will be focused on consistency of elements, and total image continuity (though I'm sure there will be some general composition elements that get added here and there ). Firstly, I have to say that I love your sky, excellent manipulation and combination of those sources, though I might have been a little more selective in the blending. Right now the lightest part of your image is the top left corner (why am I seeing stars through a heavenly body? -) which is good, it sets up for an overall image gradient to the bottom right hand corner ... except that the darkest corner is the bottom right hand corner. Since the gradient is not achieved, and nothing quite balances the white (though those clouds in the center almost achieve this, drawing attention to the character, good work on that) instead this serves to lead of the page on those white edges. One option to fix this is simply to play up the darkness of that planet, darken it up a little bit, and shade down the white along the borders where it intersects the edge of the page (sort of like a really localized vignette) (and you'd have to darken up the left edge of the page where the nebula intersects, for the same reasons). Then we have a strong dark shape in the corner that balances the white and throws us back into the rest of the image. What's great about the sky is that it is all of the same quality level, you have integrated those different images into a cohesive whole.
The water is alright, but that centered horizon line is pretty static. I think what disturbs me about the water is that, despite the obviously limited atmosphere (with seeing all those stars and planets, and the lack of atmospheric perspective) and strong overhead lighting, so much of the water is dark, with only a small portion of bounced light actually visible. The shapes of value that do exist in the water do nothing to lead the eye to the figure, they simply serve as a dividing point between the upper and lower parts of the image.
Now, the plane of the figure itself. The tree is an interesting choice, it serves the purpose of bridging the two halves, but doesn't do much for the overall composition beyond that point. If anything it stops the eye from developing a relationship between the character and the eclipse. I think the primary thing that is distracting me is the lighting on this scene. The tree has a distinct cast shadow indicating that the light is directly in front of us, but the shadow on the figure goes to the right, indicating that his light source is the eclipse (matched by the shadow on the rock) (though the backlighting of the figure shows him to be facing his light source, the same as the tree). The overall effect is to indicate that the light source is something very close, and located between the tree and figure, but this is not the case, in fact it is just inconsistent lighting. Also, why is the quality of light so different on the figure? the light on the stone is really dodged out, a lot, but this light is not seen on any of the ground or tree, as well as appearing to come even further from the left). It's okay to have one element in direct light, and everything else in shadow (check out Tintoretto's version of the Last Supper), but there needs to be some indication of why this is the case. We need a strong gradient, or some rays of light, or a distinct light source.
As it stands, your image is very beautiful, but the dischords of lighting and composition distract from the overall aesthetic. Your image has to be able to read instantaneously at any level to remain strong enough to actively engage the viewer.
Now I realize that a lot of this is the difficulty of using stock, even so, make your imagery and quality consistent.