function displaySongBackup(element, song) {
    if (_config2.default.active_index == song) {
      element.style.backgroundImage = "url(" + _config2.default.active_metadata.cover_art_url + ")";
    }
  }
Displays the song backup which is the cover art of the image in the
visualization container.

@param {node} element  - The element we are adding the background image to.