title: Merge videos and images using ffmpeg
body: <p>I'm trying to compile one .webm file that contains this:</p>
  
  <ol>
  <li>10 seconds showing image1.jpg</li>
  <li>Show a movie (an .mp4 file), which lasts about 20 seconds</li>
  <li>10 seconds showing image2.jpg</li>
  <li>10 seconds showing image3.jpg</li>
  </ol>
  
  <p>I was unable to find out how/if the concatenate functionality of ffmpeg could do such a thing. Any clues?</p>