I'm no designer but I was able to create my first app icon the other day using Inkscape. It was simple. But then it wasn't clear how to use it to generate rasterized app icons for the store. I asked a few iOS app developers if they knew of a script to do it automatically and all of them reported they did it manually with image editing software. Fooy I said, and created a tutorial called Automating iOS App Icon Creation which allows you to leverage Inkscape on Windows or Mac to generate images using a 50-line bash script. I wager the script could easily be modified to handle other native apps as well. Hope it helps someone!