I think you need to work on the form a bit more, and depth. I havn't tried airbrushing in Fireworks, but im sure you'd be able to produce a better effect in Photoshop.
I just use default brushes, and keep everything at default settings except opacity which i usually put at 1%, which i think is sufficient since my wacom controls most of the pressure.
trymake your own brush instead of using default brushes. Also try observe a real cloud and watch the shadows under it or grey places
Maybe for grunge it's good to make your own brushes, or texture work. But for digital painting, most people use default brushes. Also if you have PS7 and up you can try to load a texture onto the brush, such as the clouds one.
Those clouds actually dont look bad... for a Vector styled image, but if you want some realistic effect, you should make em a lot more fluffy, use some gray shadows and all that.
FabulisoAfrikan, you should trim down that footer, max size 70 x 40 px