Add image under the video with ffmepg
Use the vstack filter:



ffmpeg -i video.mp4 -i image.png -filter_complex vstack output.mp4


If the image is not the same width as the video then resize it with the scale filter:

ffmpeg -i video.mp4 -i image.png -filter_complex "[1:v]scale=320:-1[bottom];[0:v][bottom]vstack" output.mp4