def to_png
      return @png_data if !@png_data.nil?
      image = minimagick_image
      image.resize '16x16!'
      image.format 'png'
      image.strip
      @png_data = image.to_blob
      raise FaviconParty::InvalidData.new("Empty png") if @png_data.empty?
      @png_data
    end
def to_png
      return @png_data if !@png_data.nil?
      image = minimagick_image
      image.resize '16x16!'
      image.format 'png'
      image.strip
      @png_data = image.to_blob
      raise FaviconParty::InvalidData.new("Empty png") if @png_data.empty?
      @png_data
    end
Export source_data as a 16x16 png