title: Image overlay in ffmpeg-python
body: <p>I am trying to add a watermark to a video using <code>ffmpeg-python</code>. My pipeline is very straightforward, but I've been unable to figure out the syntax:</p>
  <pre><code>new_input = ffmpeg.input(file_name).filter('overlay', 'overlay.png').output(fout).run()
  </code></pre>
  <p>This throws the error <code>Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_overlay_7</code> , likely because the syntax for the overlay filter is incorrect.</p>