models.py

class Photo(models.Model):
    thumbnail = models.ImageField(...)
    
    def get_thumbnail(self, size='medium'):
        """Get an appropriate size thumbnail.
        
        https://example.com/image.jpg would now be
        https://example.com/image_medium.jpg
        """
        url_parts = self.thumbnail.url.rsplit('.')
        return '{}_{}{}'.format(url_parts[0], size, url_parts[1])