Hm, very simple.
I might be a little generous with the seven, but it's probably worth more of a 6.5 in my opinion. :P It looks like something I could probably create in paint or flash with very simple tools. It's also something that I could probably draw with markers, color pencils, and paper in about 20 minutes. So effort... I can't really comment on that because I don't know how much you actually did put into this piece, but to me personally it doesn't seem like a lot.
So on to what I think you could do to make it better. I think that making the stars a lot smaller (almost like your smallest one) would really add a lot to this submission. Randomly placing them around the picture and increasing the quantity would help as well. The idea is not to put too many on, but to put enough on to be effective.
I also recommend moving the moon to the upper right hand corner. Right now it seems too small, but if you were to decrease the size of the stars it would prevent you from having to increase the size of the moon as it'll just look bigger in comparison.
The bats are alright, but I have one issue with them. The bone part of the wings should be connected by a somewhat transparent skin like a real bats would be. The wings that they have here would make them fall right to the ground as it wouldn't push any air around at all. :P
Anyway, a nice little style, but it could use some detail and a few odds and ends here to make it a much better submission.