renderThumbnail() is called to render a thumbnail of the DP.
        We only render the first image (in case of multiple images)

def renderThumbnail(self, relpath=""):
        """renderThumbnail() is called to render a thumbnail of the DP.
        We only render the first image (in case of multiple images)
        """
        # return from cache if available
        cachekey, html = self.checkCache('Thumbnail', relpath)
        if html is not None:
            return html
        # else regenerate
        rec = self.imgrec[0]
        html = self._renderSingleImage(rec.fullimage, rec.thumbnail, relpath)
        # save to cache
        return self.writeCache(cachekey, html)