engine/config/initializers/dragonfly.rb

require 'locomotive'

unless Locomotive.engine?

  require 'dragonfly'

  ## initialize Dragonfly ##

  app = Dragonfly[:images]
  app.configure_with(:rails)
  app.configure_with(:imagemagick)

  ## configure it ##

  Dragonfly[:images].configure do |c|
    # Convert absolute location needs to be specified
    # to avoid issues with Phusion Passenger not using $PATH
    convert = `which convert`.strip.presence || "/usr/local/bin/convert"
    c.convert_command  = convert
    c.identify_command = convert

    c.allow_fetch_url  = true
    c.allow_fetch_file = true

    c.url_format = '/images/dynamic/:job/:basename.:format'
  end

end