$ brew install imagemagick
$ time /opt/local/convertappicons/convertappicons.sh AppName.png
AppName
real 0m4.272s
user 0m5.478s
sys 0m0.547s
$ exa -T AppName
AppName
├── Icon-60.png
├── [email protected]
├── [email protected]
├── Icon-72.png
├── [email protected]
├── Icon-76.png
├── [email protected]
├── [email protected]
├── [email protected]
├── Icon-Small-50.png
├── [email protected]
├── Icon-Small.png
├── [email protected]
├── [email protected]
├── Icon-Spotlight-40.png
├── [email protected]
├── [email protected]
├── Icon.png
├── [email protected]
├── iTunesArtwork
└── iTunesArtwork@2
$ open AppName
Then drag all the files from the newly opened folder on to your app icon grid.