Iconoclast

Iconoclast allows you to drag a base icon (1024x1024 ideally) to the window and then will generate a series of icon sizes required for app submissions. From version 1.11 the ability to alter the inbuilt icons sizes to whatever variants you want has been added. This allows the app to be changed to produced new icon sizes without the need to wait for the app to be updated. (see the menu bar options for use) Version 1.13 adds features for non square resultant images and allows the icon to be positioned within the frame (via XML).