How to add zoom transition effects using FFmpeg to a variable number of images?
I originally thought I could handle this one, but I'm still new to FFmpeg and don't think I can solve this for you. Below follows what I came up with. Maybe that can help you move in the right direction. Sorry I couldn't be more helpful.



I found the following in a search: 


  You can make a slideshow with crossfading between the pictures, by using a combination of the zoompan and framerate filters. "A" is the duration in seconds how long each picture is shown (without the crossfade duration), and "B" is the crossfade duration in seconds.


ffmpeg -i IMG_%3d.jpg -vf \
zoompan=d=(A+B)/B:fps=1/B,\ 
framerate=25:interp_start=0:interp_end=255:scene=100 \
-c:v mpeg4 -maxrate 5M -q:v 2 out.mp4


From the Superuser post you link we find:

-filter_complex \
"[0:v]zoompan=z='if(lte(zoom,1.0),1.5,max(1.001,zoom-0.0015))':d=125,fade=t=out:st=4:d=1[v0]; \
 [1:v]zoompan=z='if(lte(zoom,1.0),1.5,max(1.001,zoom-0.0015))':d=125,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1[v1]; \
 [2:v]zoompan=z='if(lte(zoom,1.0),1.5,max(1.001,zoom-0.0015))':d=125,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1[v2]; \
 [3:v]zoompan=z='if(lte(zoom,1.0),1.5,max(1.001,zoom-0.0015))':d=125,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1[v3]; \
 [v0][v1][v2][v3]concat=n=4:v=1:a=0,format=yuv420p[v]" -map "[v]" -s "800x450" -t 40 ./out_fade.mp4


Looking at the syntax, it seems you'd just toss it in without the [iN]...[oN] "variables".

So try: 

-filter_complex \
"zoompan=z='if(lte(zoom,1.0),1.5,max(1.001,zoom-0.0015))':d=125,fade=t=out:st=4:d=1"