Reduce a video to 720p:

export inn=<input.file>
ffmpeg -i $inn -vf "scale=-2:720" -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k output.mp4

Save the first frame of a video for a thumbnail:

ffmpeg -i $inn -frames:v 1 first-frame.jpg

.