Making My Own Background Image

I wanted to get a similar design and use it as an art study. I thought it would be a good test of my illustrator skills. Here's a test example below...

I liked the results. The ground plane was made using a gradient fill. Only a slight colour change was required to give it the depth it needed. I then tried to add some shadow to suggest cliffs and bumps in the terrain however some more consideration would have to be made for the position of the moon/sun. This would give it a much better sense of scale and depth.

For the final game, I want to use parallax scrolling in order to make the platforms more obvious and give a good feeling of movement. It will make it more interactive. This explains the lack of trees and clouds etc in the above test as they will be on another layer in flash. 

This is the image I create. Easy variation can be applied to the mountains etc.