user17089: I have on one side a video (of someone giving a talk) and on the other side a list of slides that were used, and the timing at where the slides were switched. I want to embed the slides in a resulting video. Do you know how to do this?
I've found how to do it with one image, but I don't know if it's possible to do it with multiple ones?
ffmpeg -i input.mp4 -i image.png \
-filter_complex "[0:v][1:v] overlay=25:25:enable='between(t,0,20)'" \
-pix_fmt yuv420p -c:a copy \
output.mp4

user1871: You can chain multiple overlay filters where each instance overlays one slide, but this will very quickly produce an unwieldy command. It's more convenient to do this in two steps. 1) Generate a video of the slides only with the required timings. 2) Use a simple ffmpeg command to overlay that video onto the main video.
For the first step, I've pasted an earlier answer I gave:

You can use a combination of two tools to do this: ffmpeg and
  mp4fpsmod
Step 1 is to generate a constant frame rate file using ffmpeg
ffmpeg -i images%d.bmp -pix_fmt yuv420p ffmpeg-cfr.mp4

Step 2 is to generate a timecode file like the one below, with each line containing the relative timestamp for a frame in
  milliseconds.
# timecode format v2

0
33
88
100
120
160
200
230
330
347

Step 3 is to use mp4fpsmod to generate the VFR file
mp4fpsmod -o vfr.mp4 -t timecodes.txt ffmpeg-cfr.mp4 

This file plays as expected with ffplay but may not with some
  players, in which case run
Step 4 Generate a CFR MP4 from the VFR using FFmpeg
ffmpeg -i vfr.mp4 final-cfr.mp4

This is a CFR file but the temporal relations are preserved as per the
  VFR, although there will be some PTS adjustments, if the timecode
  intervals are very irregular. That can be remedied by specifying a
  high framerate -r N

You'll have to rename your slide images so that their names form a continuous regular sequence. And for your case, I'd suggest your Step 1 command should be 
ffmpeg -i images%d.png -c:v libx264rgb -pix_fmt rgb24 ffmpeg-cfr.mp4

and Step 4 if needed should be
ffmpeg -i vfr.mp4 -crf 16 -c:v libx264rgb final-cfr.mp4

You can then overlay the latest file onto the lecture video, using a command like the one in your question.

17089: <p>I have on one side a video (of someone giving a talk) and on the other side a list of slides that were used, and the timing at where the slides were switched. I want to embed the slides in a resulting video. Do you know how to do this?</p>

<p>I've found how to do it with one image, but I don't know if it's possible to do it with multiple ones?</p>

<p><code>ffmpeg -i input.mp4 -i image.png \
-filter_complex "[0:v][1:v] overlay=25:25:enable='between(t,0,20)'" \
-pix_fmt yuv420p -c:a copy \
output.mp4</code></p>
 1871: <p>You can chain multiple overlay filters where each instance overlays one slide, but this will very quickly produce an unwieldy command. It's more convenient to do this in two steps. 1) Generate a video of the slides only with the required timings. 2) Use a simple ffmpeg command to overlay that video onto the main video.</p>

<p>For the first step, I've pasted an earlier <a href="https://stackoverflow.com/q/28086775/5726027">answer</a> I gave:</p>

<blockquote>
  <p>You can use a combination of two tools to do this: ffmpeg and
  <a href="https://stackoverflow.com/q/28086775/5726027">mp4fpsmod</a></p>
  
  <p><strong>Step 1</strong> is to generate a constant frame rate file using ffmpeg</p>

<pre><code>ffmpeg -i images%d.bmp -pix_fmt yuv420p ffmpeg-cfr.mp4
</code></pre>
  
  <p><strong>Step 2</strong> is to generate a timecode file like the one below, with each line containing the relative timestamp for a frame in
  milliseconds.</p>

<pre><code># timecode format v2

0
33
88
100
120
160
200
230
330
347
</code></pre>
  
  <p><strong>Step 3</strong> is to use mp4fpsmod to generate the VFR file</p>

<pre><code>mp4fpsmod -o vfr.mp4 -t timecodes.txt ffmpeg-cfr.mp4 
</code></pre>
  
  <p>This file plays as expected with <code>ffplay</code> but may not with some
  players, in which case run</p>
  
  <p><strong>Step 4</strong> Generate a CFR MP4 from the VFR using FFmpeg</p>

<pre><code>ffmpeg -i vfr.mp4 final-cfr.mp4
</code></pre>
  
  <p>This is a CFR file but the temporal relations are preserved as per the
  VFR, although there will be some PTS adjustments, if the timecode
  intervals are very irregular. That can be remedied by specifying a
  high framerate <code>-r N</code></p>
</blockquote>

<p>You'll have to rename your slide images so that their names form a continuous regular sequence. And for your case, I'd suggest your <strong>Step 1</strong> command should be </p>

<pre><code>ffmpeg -i images%d.png -c:v libx264rgb -pix_fmt rgb24 ffmpeg-cfr.mp4
</code></pre>

<p>and <strong>Step 4</strong> if needed should be</p>

<pre><code>ffmpeg -i vfr.mp4 -crf 16 -c:v libx264rgb final-cfr.mp4
</code></pre>

<p>You can then overlay the latest file onto the lecture video, using a command like the one in your question.</p>
