To create the MacOS icon, do
* In GIMP, export the icon to tiff (png might be as good or better), fullsize (currently 128x128 [Aug 08])
* You should also export a 16x16 icon to the same format, tweaked for readability at that small size
* Go to a mac machine
* run 'Icon Composer' and create the icon from the exported tiff
	* drag fullsize tiff from finder to the appropriate panel in Icon Composer (currently the 128x128 panel)
	* it should ask about copying the image to different sizes, opt for 'Copy to all smaller sizes'
	* drag the 16x16 image to the 16x16 panel
	* exit Icon Composer
	* you will be asked where to save, save to shared/core/icon/icon.icns


To create the Windows icon, do
* In GIMP, export icons of these sizes: 128x128, 64x64, 48x48, 32x32, 16x16
* 16x16 icon should have the edges cropped for readability at this small size
* Open each as a layer in a new GIMP image (File->Open as Layer), and delete other layers (like 'Background')
* Save as an ico file


To create .ico (Widnows) to .icns (Mac), visit the online converter at http://iconverticons.com/online/
Browse to the file, press Options, tick both the icns option and the 32 bit option.

http://brettterpstra.com/2011/03/05/grabbing-a-mac-apps-icon-building-blocks/
Finding an app's icon with the defaults command
On a Mac, every application is actually a folder, referred to as a “bundle”. Within that folder are libraries, executables and resources (like images and icons). There's also an important file called Info.plist which stores information like the application’s name, identifiers, file types and–most relevant right now–the name of the file containing the application’s icon. 
		The application icon is an an .icns file, stored in AppName.app/Contents/Resources. 
Its title does not have to be the same as the app’s, so–before we can extract and convert it–we need to get its actual filename from Info.plist.