Back in the day, before Apple unleashed the new generation of iMove onto unsuspecting Mac users, iMove made it easy to extract a freeze frame, still image from a movie clip.
For whatever reason, Apple has deemed that functionality persona non grata and the whole freeze frame process in the new iMovie is convoluted. Or, you can use SnapMotion which grabs single frames of video with a click. This is one of those one-trick pony utilities that doesn’t do much, but does exactly what you want.
Drag and drop a video clip onto SnapMotion and it pops up what looks like a QuickTime Movie Player window but with an empty timeline at the bottom.
Drag the movie slider bar to the point in the movie clip where you want to create a freeze frame snapshot, and click the camera snapshot button.
SnapMotion moves the freeze frame photo to the timeline. Repeat as many times as needed to capture many photos from a video clip.
Clips can be capture manually using the method above, or captured automatically every x-number of seconds. Images can even be resized before being saved. It just isn’t easier to capture a freeze frame image in a movie clip unless Apple builds the functionality back into iMovie.