How to set apple icon image (xxx.icns) to file or folder

I think I've found a workaround!

First off, install osxutils bundle which is a bit dated (2005) but nevertheless it works. (see the osxutils contents and man pages)

Prepare the PNG to ICNS^†—ironically you can use sips^‡. Then use seticon from the osxutils bundle to change the icon:

$ sips -s format icns FIN.png --out FIN.icns
$ seticon -d FIN.icns FIN.png

The seticon also changes the file's attributes, ie. using SetFile -a C isn't necessary.

†: Using a PNG as a source file for seticon changes the icon to a generic PNG icon, even with -d option. Furthermore, using seticon FIN.icns FIN.png changes the icon to a generic ICNS icon. (Consult the seticon's man pages)

‡: NB: the source file must be a square and possible side-lengths are: 16, 32, 48, 128, 256 and 512.

Any image can be used to create an icon for a folder. The problem is that you're trying to copy the image from the Finder's preview. Not all images are saved with a preview like that, so you can't always use that as a source.

Fortunately, there's a good way around that. When you copy that icon, all you're copying is the image data. You can also do that by opening any image in Preview, selecting all (cmd+a), and copying. Go to your folder's Get Info window, select that icon at the top right, and paste.

I tested, and this works on Lion.