Return the relative path of the thumbnail.

        path:
            path of the source image
        key:
            key of the thumbnail
        format:
            thumbnail file extension

def get_thumbpath(self, path, key, format):
        """Return the relative path of the thumbnail.

        path:
            path of the source image
        key:
            key of the thumbnail
        format:
            thumbnail file extension
        """
        relpath = os.path.dirname(path)
        thumbsdir = self.get_thumbsdir(path)
        name, _ = os.path.splitext(os.path.basename(path))
        name = '{}.{}.{}'.format(name, key, format.lower())
        return os.path.join(relpath, thumbsdir, name)