code: def renderThumbnail(self, relpath=""):
          
          # 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)
docs: renderThumbnail() is called to render a thumbnail of the DP.
          We only render the first image (in case of multiple images)