Needed because the image is looping indefinitely. t 10 (in the first example) makes a 10 second output.

If your input is already AAC or M4A then use -c:a copy instead to stream copy instead of re-encode. Recommended for libx264 when encoding from images. This ensures the output uses a widely acceptable chroma sub-sampling scheme.

vf format=yuv420p outputs the pixel format as yuv420p. Use the highest value that still provides an acceptable quality to you. tune x264 tuning for various adjustments to fit specific situations. Some players have issues with low frame rates so a value over 6 or so is recommended. framerate sets the frame rate of the image input. The output duration will match the input audio duration: ffmpeg -loop 1 -framerate 24 -i input.jpg -i audio.mp3 -c:v libx264 -preset slow -tune stillimage -crf 24 -vf format=yuv420p -c:a aac -shortest -movflags +faststart output.mp4

Example for a 10 second output: ffmpeg -loop 1 -framerate 24 -i input.jpg -c:v libx264 -preset slow -tune stillimage -crf 24 -vf format=yuv420p -t 10 -movflags +faststart output.mp4

Ffmpeg transcode to h264