How to embed multiple images in a video recording
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.