a.jpg = 26.7K (image not so clear)
a.mp3 = 64.6M (54 minutes)
a.mp4 = 80.6M (video result is not so clear, music still good)

$ ffmpeg -loop 1 -i a.jpg -i a.mp3 -shortest -acodec copy a.mp4

a.jpg = 151K
a.mp3 = 5.1M (5 minutes)
a.mp4 = 9.2M

ffmpeg -loop 1 -framerate 1 -i image.jpg -i music.mp3
-c:v libx264 -preset veryslow -crf 0 -c:a copy -shortest output.mkv