Automatically Create Icons for iOS Apps. Full credits to Yuya Kitajima (aka @yuyak on Github) for this rake task, shared as a Coderwall tip. https://coderwall.com/p/otmx2g

Install imagemagick

brew install imagemagick

Rakefile

# coding: utf-8

desc 'Create icons'
  task :create_icons do

  # Source icon filename
  input = 'Icon-1024.png'
  output_dir_path = 'Images/'

  [
    { name: 'Icon-72.png', size: 72 },
    { name: 'Icon-72@2x.png', size: 144 },
    { name: 'Icon-Small-50.png', size: 50 },
    { name: 'Icon-Small.png', size: 29 },
    { name: 'Icon-Small@2x.png', size: 58 },
    { name: 'Icon.png', size: 57 },
    { name: 'Icon@2x.png', size: 114 },
  ].each do |v|
    output = output_dir_path + v[:name]
    command = "convert -resize #{v[:size]} #{input} #{output}"
    puts command
    system command
  end
end

Run the rake task

rake create_icons