Very cool composition! With more contrast and accurate lighting this piece could be a real stunner. The main thing is the lack of dark areas on the asteroids, causing them to look a bit dull and uninteresting. The nebulae as well need some contrast boost.
The blur seems conflicting as this is an otherwise serene spacescape. And it seems pointless and bossy to restrict your own artwork, including a watermark that would look hideous on a desktop. Any artist should be honoured to have their work distributed if it wasn't going to earn them money.