Isn't it supposed to depict the letter "d" or "D"? I see neither.
I thought it was fairly obvious.
You've got a few different shades of green and you need to use them smarter. Create some smoother transitions with that blueish green. It might not hurt to push those greens into a more yellow direction.
Mine < - > Yours
(http://img269.imageshack.us/img269/1601/saurg.png)(http://img29.imageshack.us/img29/1601/saurg.png)
I played inside of your lines, with your colors just to show you where you can drop a few black pixels and smooth out a few things.
USE REFERENCES!