Your Saber sigs and avatars are really good!!!!!! I recommend to use Gradient tool with a really dark blue to light blue or white for your sky in your wallpaper. Also, use different colors of green (mostly a dark to regular green). For cloud, try to look for brushes on graphic site. This might help to improve your wallie. I maybe not be that experienced but it help me. Don't worry, I screwed up on my first wallpaper with grass too. Here you can see.
Anyways, keep up with the great work!!!!!