Recently while in the process of preparing an application for Apple Mac Store, it occurred to me that it was not immediately obvious how to prepare an application's icon correctly for submission.
Apple's submission guidelines stipulate; that for a Macintosh application destined for the Mac Store it needs a 128x128 icon and a 512x512 sized icon. Adding further to confusion are the differences in requirements between an iPad Application and a standard Macintosh Application. For an iPad application the application's icon and the application's iStore icon are submitted as part of the application bundle and importantly the icon and the iStore icon are included separately within the bundle.
In a Macintosh application the two icons are stored together in a single file. Apple's icns format handily allows the storing of multiple resolutions in one file. In order to generate the requisite file you can use Icon Composer (pictured below).
Starting with two png files, one 128x128 pixels and one 512x512 pixels in size - one needs to first convert them to icns format. You can do this at the command line using the handy sips binary shipped with OSX.
sips -s format icns 128x128.png --out 128x128.icns sips -s format icns 512x512.png --out 512x512.icns
Once we have our icons file in icns format we can fire up the Icon Composer. Open the 512x512 and 128x128 files in Icon Composer. Drag the 128x128 image into place, the outlined box reserved for that size image with in the 512x512 window and voila the two are linked. You can save out that resulting file to a new icns file and use this in your Xcode project.