Um...first, when posting in the Sandbox, please post a link to the image rather than the entire image.
* Contrary to what Shujaa said, adding a simple stock photo for the bg won't work. Using an unedited, or poorly
edited, stock photo for a significant portion of the bg is not allowed here at MT.
* The bg definitely has potential. Judging from the bright colors of the chara, I would say day scene. However, it looks
like you've already decided on night scene, so we'll just go with that. A/some moon(s), stars, and clouds seem
to be in order. There are plently of tutorials for planet and star effects, so that should work out.
* There's a lot of blank space here, so you should fill in more of the bottom to avoid bottom-heaviness. Like
Sentinal said, some mountains in the bg would be a good idea for that.
* Text! Of course most walls need text, and text gives the wall a lot of structure and character. With all that space,
you could go in for some pretty dramatic text.
Hope this helps. :)