Originally posted by @maartenbreddels in #392 (comment):

I plan to manually take screenshots/screen captures of ipyvolume. I'd like to thus manually add the thumbnail. If I do this in conf.py, I cannot add a tooltip.

• Would it be an idea to add this as well, .e.g:

  nbsphinx_thumbnails = {
      'examples/scatter': 'examples/scatter.png',
      'examples/volshow': {'filename': 'examples/scatter.png', 'tooltip': 'Basic volume,
  }
  

• Even better would be if we could auto generate a tooltip from the 1st markdown cell, the first sentence after the first H1 (i guess that is what sphinx-gallery does).