1. Open Terminal / Command Prompt
  2. Change directory to the location of you video with cd c:\your\directory\here (cd /your/directory/here on Mac)
  3. Use the following command (with red text modified for your situation)

ffmpeg -i INVIDEO.MP4 -ss [time] -f image2 -vframes 1 OUTIMAGE.BMP

Details: Change INVIDEO.MP4 and OUTIMAGE to your video file and output file, replace [time] with the time of the still frame you want to save as a still. [time] can be in seconds (24.4), or it can be in the format hh:mm:ss.xxx (00:00:24.400).

Tip: open up Notepad, Word or your favorite text editor and copy/paste this command into a blank sheet. You can edit it there to fit your needs, then copy/paste it into the Command Prompt/Terminal.


Examples


I have a 90 second FAST done by a resident (fast.avi). I want to extract a still image of a Morison's Pouch view at 75.5 seconds in. I will use the following command:
ffmpeg -i fast.avi -ss 75.5 -f image2 -vframes 1 MP.BMP
alternately, I could use this format to get the same result:
ffmpeg -i fast.avi -ss 00:01:75.500 -f image2 -vframes 1 MP.BMP

You can combine this with cropping by adding the cropping flag, read more here. Lets say I have a 15 second GB ultrasound done by a resident (GB.mov). I want to extract a still from this file at 10 seconds, cropping off top 28 pixels of patient info. I will use the following command:
ffmpeg -i GB.mov -ss 10 -vf crop=in_w:in_h-28:0:28 -f image2 -vframes 1 GB_still.BMP