There are two options, for you to choose:
1. Set fps to 1, and use PNG sequence, so it will only create frames every second
2. Use this command line setting: -y -f rawvideo -pix_fmt rgb24 -s %WIDTH%x%HEIGHT% -r 1 -ss 1 -t 1 -i - "%FILENAME%.png"
(Put it in the right text field, at the command line settings tab.) This will only export the first frame BUT it will crash minecraft