ffmpeg version 3.2.10-1~deb9u1~bpo8+1 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 4.9.2 (Debian 4.9.2-10)

/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"

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

2. 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.

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

How should I change the filter_complex?