I can't seem to find Icon Composer on the Xcode versions at run on Mountain Lion.
Where is it? Has it been taken out? How to get it back?
It's been moved into one of the add-on packages for Xcode, and is now part of the Graphics Tools for Xcode package.
Apparently this is (at least partly?) because this is no longer the 'officially recommended' way to generate (high resolution) icons. See the OS X Human Interface Guidelines and the High Resolution Guidelines for more info.
I believe Icon Composer doesn't support the highest resolution icons required these days (1024x1024), and you're encouraged to use
If you still want it, you can get it as follows:
In Xcode, go to the Xcode menu, then Open Developer Tool and select More Developer tools....
You'll need to log in (or register) with your (free) Apple Developer account, and then you'll be shown a list of available packages for Xcode, including the Graphics Tools for Xcode. Grab the latest one and install.
This DMG of Graphics Tools contains:
Actually you are mistaken. Icon Composer in its latest version does support the 1024x1024 size. Just go to the menubar and go to view > Show 1024x1024 or click the 1024x1024 button at the bottom of the window. I have made icons with that size before.
What I found easier than downloading, installing, and finding Icon Composer was using the command-line tool
You must have the following set of pngs: