has_attached_file :source,
  :styles => {
    :max => ['600x600>', :jpg],
    :large => ['400x400>', :jpg],
    :medium => '300x300>',
    :small => '200x200#'
  },
  :convert_options => {
    :all => "-strip",
    :max => '-background white -flatten -quality 75',
    :large => '-background white -flatten -quality 75'
  }