To make an audio aac file from a downloaded youtube mp4 video:
I put this function in my .bash_profile:

vtoa () 
    /usr/bin/ffmpeg -i "$1" -vn -acodec copy "${1%.*}".aac

To convert I just have to type vtoa videofilename.mp4

To convert folder of numbered pics to mp4: e.g.

ffmpeg -f image2 -r 15 -i /my/path/00%d.png -vcodec h264 -crf 15 test.mp4

(that’s 15 frames per second, filenames with 2 leading zeroes, starting with 000.png, constant rate factor 15 (lower is better)..)
That’s for filenames like 001.png, 00100.png, 001000 etc.
For 0-padded names, 001, 010, 100 etc use %03d.png

To convert image files in a folder to consecutive filenames, and convert to jpeg:

$ f=-1;for i in /my/path/*
> do f=$((f+1))
> echo -n sips -s format jpeg "'"$i"'"
> printf " --out /my/path2/%05d.jpg\n" $f
> done > ex
$ . ex