def thumbnail(self):
        """
        Display thumbnail-size image of ImageField named src
        Assumes images are not very large (i.e. no manipulation of the image is done on backend)
        Requires constant named MAX_THUMB_LENGTH to limit longest axis
        """
        max_img_length = max(self.get_src_width(), self.get_src_height())
        ratio = max_img_length > MAX_THUMB_LENGTH and float(max_img_length) / MAX_THUMB_LENGTH or 1
        thumb_width = self.get_src_width() / ratio
        thumb_height = self.get_src_height() / ratio
        url = '%s%s' % (settings.ADMIN_MEDIA_PREFIX, self.get_src_url())
        return '<img src="%s" width="%s" height="%s"/>' % (url, thumb_width, thumb_height)
    thumbnail.short_description = 'Image thumbnail'
    thumbnail.allow_tags = True

    class Admin:
        list_display = ('src', 'thumbnail',)