You can also use VLC Media Player which is free, you can vary the speed of playback (i.e slow motion) then pause the playback when you get the frame you want and then take a snapshot. It will save the frame as a .png file.
There are some other tricks as well, if you overshoot the frame you want you can get VLC to go back 5 or x amount of frames and then go forward again. But its best to do this in slow motion as its easy to overshoot, especially if the video is 25 frames per second.
NB: Be careful & do not select "Every frame" otherwise you'll end up with a very full folder.
I found it best to "snip/cut" your selected section of the video to just a few minutes.