To take a screenshot or save a frame as an image with FFmpeg here is the basic command:
ffmpeg -ss 01:10:35 -i invideo.mp4 -vframes 1 -q:v 3 screenshot.jpg
-ss is called seeking in FFmpeg terms.
The above command will take the screenshot at 1 hour, 10 minutes and 35 seconds.
-q:v is quality. On a range from 1-31 with the 1 being best, 31 worst.
-vframes 1 means the frame number at the time specified.
You can shorten the code to not have the quality filter:
ffmpeg -ss 01:10:35 -i invideo.mp4 -vframes 1 screenshot.jpg
There it is, simple and quick with how to take a screenshot in FFmpeg.