code: def test_download_image(self):
      # Not download image by default
      html = plotly.offline.plot(
          fig_frames, output_type="div", auto_play=False
      )
      self.assertNotIn(download_image, html)
  
      # Request download image
      html = plotly.offline.plot(
          fig_frames, output_type="div", auto_play=False, image="png"
      )
      self.assertIn(download_image, html)
  
imports: import json
  import os
  from unittest import TestCase
  import pytest
  import plotly
  import plotly.io
  from plotly.io._utils import plotly_cdn_url
label_desc: Use image node_id 28 for calling the PlotlyOfflineTestCase obj's underlying member method code with example usage: obj.test_download_image() without return types