def stdimg(instance, image, thumbnail, force_size=False):
  '''
    standarization of image fields (for being used when saving models)
      automatically creates thumbnail
      change image name to /path/to/images/<my_field>-<id>.<ext>
      resize image to specified size
    arguments:
      instance: model instance to be saved
      image: tuple containing image field name, and desired size (a tuple containing width and height)
      thumbnail: tuple containing thumbnail field name, and desired size (a tuple containing width and height)
    example:
      class MyModel(models.Model):
        my_image = models.ImageField(upload_to='path/to/images')
        my_thumbnail = models.ImageField(upload_to='path/to/thumbnails', editable=False)

        def save(self):
          super(MyClass, self).save() # We force to save and create an object id to use in image name
          stdimg(self, ('my_image', (640, 480)), ('my_thumbnail', (100, 75)))
          super(MyClass, self).save() # We update paths on db
  '''
  import os
  from django.conf import settings
  from PIL import Image
  src = eval('instance.get_%s_filename()' % image[0])
  if src:
    ext = os.path.splitext(src)[1].lower().replace('jpg', 'jpeg')
    filename = lambda s: instance._meta.get_field(s).get_filename('%s-%s%s' % (s, instance.id, ext))

    for field in (image, thumbnail):
      img = Image.open(src)
      if force_size:
        size = field[1]
        ratio = lambda t: float(t[1]) / float(t[0])
        if ratio(size) > ratio(img.size):
          x, y = float(img.size[0]) / img.size[1] * size[1], size[1]
          pos_x, pos_y = float(x - size[0]) / 2, 0
        else:
          x, y = size[0], float(img.size[1]) / img.size[0] * size[0]
          pos_x, pos_y = 0, float(y - size[1]) / 2
        img = img.resize((x,y), Image.ANTIALIAS)
        img = img.crop((pos_x, pos_y, pos_x + size[0], pos_y + size[1]))
      else:
        img.thumbnail(field[1], Image.ANTIALIAS)
      img.save(settings.MEDIA_ROOT + filename(field[0]))
      setattr(instance, field[0], filename(field[0]))

    if os.path.exists(src) and src != settings.MEDIA_ROOT + filename(image[0]):
      os.remove(src)