listen(el) {
    if (typeof el === 'string') {
      const els = document.querySelectorAll(el)
      let i = els.length

      while (i--) {
        this.listen(els[i])
      }
    } else if (el.tagName === 'IMG') {
      el.style.cursor = cursor.zoomIn
      listen(el, 'click', this.handler.click)

      if (this.options.preloadImage) {
        loadImage(getOriginalSource(el))
      }
    }

    return this
  }
Make element(s) zoomable.
@param  {string|Element} el A css selector or an Element.
@return {this}