I have created a directory with the icon set extension and put some png file there. After typing the below commands I have been unable to generating icon files.
$ mkdir myicon.iconset
$ iconutil -c icns myicon.iconset
$ Failed to generate ICNS.
The files inside of myicon.iconset need to have a particular naming structure, e.g.:
icon.iconset/icon_128x128.png
icon.iconset/icon_128x128@2x.png
icon.iconset/icon_256x256.png
icon.iconset/icon_256x256@2x.png