2 tools for creating many sizes of app icons when building your mobile app
Creating all of the appropriate sized icons for your app is definitely a pain. There are websites and tools that exist to automate the process.
A STANDALONE APP
iconbot (formerly known as icon robot) is a standalone app that provides a nice UI for this process and allows you to select which app stores you’re building them for.
A PHOTOSHOP SCRIPT
Beadpond provides an excellent Photoshop script to create app icons for you. I like this script because it’s open source so you could customize it to suit your needs.