How do I add a custom thumbnail to a .mp4 file using ffmpeg?

Question

I am trying to find the proper ffmpeg command to add a .png or .jpg image to a .mp4 video as a thumbnail. The command will eventually automatically be executed by a C# prog

Answer 1

Using ffmpeg 4.0, released Apr 20 2018 or newer,

ffmpeg -i video.mp4 -i image.png -map 1 -map 0 -c copy -disposition:0 attached_pic out.mp4