FFmpeg: Convert mp3 & album art to video in one step?
I know that you were trying to find a way to do this in a single step. However, for those who stumble across this question, it's much easier to chain two ffmpeg commands like so:

ffmpeg -i in.mp3 -an out.png &amp;&amp; ffmpeg -loop 1 -i out.png -i in.mp3 -c:a aac -ab 256k -c:v libx264 -pix_fmt yuv420p -shortest -strict -2 out.mp4


Note that this reencodes the mp3, as it's not a supported codec in mp4 containers.