Extract frame from video (replace x with desired frame number)
ffmpeg -i <input> -vf "select=eq(n\,x)" -vframes 1 poster.png
Recompress MP4 (replace x with desired bytes/s e.g. 10000)
ffmpeg -i <input>.mp4 -b x <output>.mp4
Convert PNG sequence to Prores 422 (expects file names incrementing from 0000000.png)
ffmpeg -r 25 -f image2 -i %07d.png -c:v prores_aw -profile:v 1 -pix_fmt yuv422p10 -r 25 .mov