code: def convert_images(image_list):
      
      png_output_contains = 'PNG image'
      ret_list = []
      for image_file in image_list:
          if os.path.isdir(image_file):
              continue
  
          dummy1, cmd_out, dummy2 = run_shell_command('file %s', (image_file,))
          if cmd_out.find(png_output_contains) > -1:
              ret_list.append(image_file)
          else:
              # we're just going to assume that ImageMagick can convert all
              # the image types that we may be faced with
              # for sure it can do EPS->PNG and JPG->PNG and PS->PNG
              # and PSTEX->PNG
              converted_image_file = get_converted_image_name(image_file)
              cmd_list = ['convert', image_file, converted_image_file]
              dummy1, cmd_out, cmd_err = run_shell_command(cmd_list)
              if cmd_err == '':
                  ret_list.append(converted_image_file)
              else:
                  raise Exception(cmd_err)
      return ret_list
docs: Convert list of images to PNG format.
  
      @param: image_list ([string, string, ...]): the list of image files
          extracted from the tarball in step 1
  
      @return: image_list ([str, str, ...]): The list of image files when all
          have been converted to PNG format.