This is how I created the main “icon” for Armageddon Wars. The main App Store icon has to be 512×512 pixels, but usually is displayed much smaller, so the image had to look good even scaled down 10 times.
The shape of a painting is what most affects the composition. If you get this right you can get a great dramatic effect, but you need to start out very abstract. So, what I do is just start drawing random abstract shapes, until I hit on one that I like:
Once you have a good shape you start adding details.
I had some tank models and city ruins done in Max, so I thought I would take advantage of that and made some test scenes in Max. This would also ensure that I got the lighting correct for the scene.
Then I imported this into photoshop, added a background of an inferno (that I took from a photo) and started working on the barrel.