def source
      return @source unless @source.nil?
      @source = %i(string template id path).find { |from| @options.key?(from) }
    end
def source
      return @source unless @source.nil?
      @source = %i(string template id path).find { |from| @options.key?(from) }
    end
Gets the source to generate the image from.

 Possible values: `:string`, `:template`, `:id`, `:path`.

 @example Building an Image from a Path
   self.source #=> :path

 @example Building an Image from a Template
   self.source #=> :template

 @return [Symbol] The source.

 @api private