First let me say that I really love the way the whole images looks like. There is a nice line that starts on the bottom right part, leading the eye through the image to the top left explosion. And the explosion itself gives a nice vibrant feeling. The blue contrasts really nice with the red/yellow tones. But I'm not sure with the starfield of the image. There should be much more stars in the area of the nebula and only a few in the rest of the image. Also the random escaping ships could get a more organized flight. It is supposed to be a wallpaper and that's why the negative space works well. With the help of empty ares, you know where to place your icons. If it would be a normal artwork I would suggest to add some more elements.

But nevertheless: nice work Jeff