How to create ICNS icons for OS XJune 18, 2012, revised March 17, 2014 ICNS OS X XCode
You can google up that it’s possible to use ImageMagick (
sips (the Apple image management utility bundled with OS X) to create ICNS icons from, say, PNG images. But even though they show up correctly in Finder, you’ll be disappointed to find out that they will not show up applied to an application or script unless all variants of the icon are present (or at least 128x128, 64x64, 32x32 and 16x16).
So imagemagick CANNOT create ICNS files. See the list of supported formats for yourself.
The tool you need is called Icon Composer, from Apple, free. It lets you create a complete ICNS icon from your PNG icon, even if you only have it in one resolution. Icon Composer was shipped with XCode some time ago, but with recent XCode releases, you have to install it separately.
If you’re not using a Mac, your best bet is to use some online ICNS conversion tool.