/usr/bin/ffmpeg -y \
   -i "INPUT-MP4" \
   -i "WM-PNG" \
-filter_complex " \
[1:v]format=rgba,colorchannelmixer=aa=0.4[fg]; \
[fg]setsar=1[logo]; \
[0][logo]overlay=10:10" \
 "output.mp4"

The above works;  and generates output.mp4 with an embedded overlay.

I now want to add an 3rd input "fullscreen.jpg"  and display this for 5 seconds in the output.mp4 at the end of the file.

Do I also need a 4th input, to generate 5 seconds of silent audio?