You can use a variety of image formats with your app: .png, .jpeg, .gif, .tiff, .bmp, .ico, and .xbm. The recommended image format is PNG; when your app is compiled, the PNG images are optimized for the iPhone.
Here are some graphics programs you can use to create your images:
|Prepo converts your @2x-sized images down to half-sized versions (for supporting retina and non-retina displays).|
|Skala Preview - view image previews of your app design on your mac or any iOS device.|
|Griddle - A simple grid generator app for the Mac.|
The App Icon Template is a PSD file that uses smart objects to automatically render your app icon in all the various sizes needed.
Dev Rocket is a Photoshop plugin that helps you create graphics and icons for your iOS apps.