oldiconutil
In Mac OS X 10.6, Apple introduced support for PNG-compressed icon data in .icns files. In Mac OS X 10.8, Apple introduced support for Retina icons and the iconutil command-line tool for generating icons from a folder of PNG files.
However, iconutil only knows how to generate PNG-compressed icon data, which means that you can't use it to create a Retina icon that will still display properly on Mac OS X 10.5.
He’s written some code that converts .icns files to use JPEG 2000 internally instead of PNG, which makes them readable on Mac OS X 10.5. Kudos to Kusterer for figuring this out, and shame on Apple for silently breaking compatibility.
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I'm very grateful that you did this.
For those of us who are not developers, please, could you take it one step further and give us a compiled tool that will run on the older OSes where the problem shows up.
That way, we can fix it ourselves, if necessary.
So maybe that's the reason why the application icon of Transmit's Panic 4.2 didn't show up on Leopard…