```ruby
# Used only for Ruby on Rails gem to tell, that gem contain `lib/assets`.
module Fotoramajs
  class Railtie < Rails::Railtie
    initializer 'fotorama.config' do |app|
      app.config.assets.precompile += ['fotorama.png', 'fotorama@2x.png']
    end
  end

  module Rails
    class Engine < ::Rails::Engine
    end
  end
end
```

Add pngs to precompile only in Rails 4
user
```ruby
# Used only for Ruby on Rails gem to tell, that gem contain `lib/assets`.
module Fotoramajs
  class Railtie < Rails::Railtie
    initializer 'fotorama.config' do |app|
      if Gem::Version.new(::Rails.version) >= Gem::Version.new("4.0.0")
        app.config.assets.precompile += ['fotorama.png', 'fotorama@2x.png']
      end
    end
  end

  module Rails
    class Engine < ::Rails::Engine
    end
  end
end
```
assistant