code: def _create_alternative_resolutions(self, source_image, geometry_string,
                                          options, name):
          
          ratio = default.engine.get_image_ratio(source_image, options)
          geometry = parse_geometry(geometry_string, ratio)
          file_name, dot_file_ext = os.path.splitext(name)
  
          for resolution in settings.THUMBNAIL_ALTERNATIVE_RESOLUTIONS:
              resolution_geometry = (int(geometry[0] * resolution), int(geometry[1] * resolution))
              resolution_options = options.copy()
              if 'crop' in options and isinstance(options['crop'], string_types):
                  crop = options['crop'].split(" ")
                  for i in range(len(crop)):
                      s = re.match(r"(\d+)px", crop[i])
                      if s:
                          crop[i] = "%spx" % int(int(s.group(1)) * resolution)
                  resolution_options['crop'] = " ".join(crop)
  
              image = default.engine.create(source_image, resolution_geometry, options)
              thumbnail_name = '%(file_name)s%(suffix)s%(file_ext)s' % {
                  'file_name': file_name,
                  'suffix': '@%sx' % resolution,
                  'file_ext': dot_file_ext
              }
              thumbnail = ImageFile(thumbnail_name, default.storage)
              default.engine.write(image, resolution_options, thumbnail)
              size = default.engine.get_image_size(image)
              thumbnail.set_size(size)
docs: Creates the thumbnail by using default.engine with multiple output
          sizes.  Appends @<ratio>x to the file name.