FFmpeg to save a video frame from a specified moment to the image, an -ss (seek from start) option is used to specify the delay from the start. The syntax for taking a screenshot in the time t is
ffmpeg -i input -ss t image.type
The -ss option can be used also before the input file, but the result is less accurate. For example to take a screenshot in the time 1 hour 23 minutes 45 seconds from the file videoclip.avi, we can use the command
ffmpeg -i videoclip.avi -ss 01:23:45 image.jpg