code: def redo(self, channel, image):
          
          self.logger.debug("image set")
          chname = channel.name
  
          # get old highlighted thumbs for this channel -- will be
          # an empty set or one thumbkey
          old_highlight = channel.extdata.thumbs_old_highlight
  
          # calculate new highlight thumbkeys -- again, an empty set
          # or one thumbkey
          if image is not None:
              thumbkey = self._get_thumb_key(chname, image)
              new_highlight = set([thumbkey])
          else:
              # no image has the focus
              new_highlight = set([])
  
          # TODO: already calculated thumbkey, use simpler test
          if not self.have_thumbnail(channel.fitsimage, image):
              # No memory of this thumbnail, so regenerate it
              self._add_image(self.fv, chname, image)
  
          self.logger.debug("highlighting")
          # Only highlights active image in the current channel
          if self.highlight_tracks_keyboard_focus:
              self.update_highlights(self._tkf_highlight, new_highlight)
              self._tkf_highlight = new_highlight
  
          # Highlight all active images in all channels
          else:
              self.update_highlights(old_highlight, new_highlight)
              channel.extdata.thumbs_old_highlight = new_highlight
docs: This method is called when an image is set in a channel.