I havent tried this on Android. But on PC you can use this to create the mp4 file using ffmpeg
The following worked for me
ffmpeg -i download.jpg -r 1/100 -i test.mp3 -s 720x480 out.mp4
https://trac.ffmpeg.org/wiki/Create%20a%20video%20slideshow%20from%20images https://trac.ffmpeg.org/wiki/Create%20a%20thumbnail%20image%20every%20X%20seconds%20of%20the%20video