ffmpeg -t 1 -f video4linux2 -s 640x480 -r 1 -i /dev/video0 foto.jpeg
если снимки будут тёмные, то –r 20
-r[:stream_specifier] fps (input/output,per-stream)
Set frame rate (Hz value, fraction or abbreviation).
-t duration (input/output)
When used as an input option (before -i), limit the duration of data read from the input file.