ffmpeg -i still.png -r .1 video.mp4Sadly this creates the 10 second video but it doesn't have the still image in it. But I'm still sharing this tip because it seems close to a solution.
Computer Tips - How can I use ffmpeg to convert a still into a 10 second video?
Date: 2017aug4 Q. How can I use ffmpeg to convert a still into a 10 second video? A. But setting the framerate really low (.1) you can get a 10 second video.