Grabbing Frames from a Video Clip
If you want to grab a single frame from your video to use as a still photo (in order to use the picture on a Web page, for example), move the Playhead to the frame you want to select.
In the menu at the top of the screen, select File...Save Frame As...
In the dialog box that comes up, type in a name for the frame and select the folder into which you want to save it.
From the Format picklist, choose jpeg.
Then click on Save.
If you want to use a single frame from a video clip in your movie - in effect freezing the motion on a particular image as your movie plays - you can grab a still frame from a clip.
To do that, move the Playhead to the frame you want to use. Then in the menu at the top select Edit...Create Still Frame.
The still frame will appear on the Shelf. By default, iMovie will create a still frame that is 5 seconds in duration (in other words, the same frame or picture will be displayed for 5 seconds in a movie).
Drag the still frame from the Shelf to the Viewer just as you would a video clip to add it to the Timeline. Once in the Timeline, you can change the duration of the still frame by editing it with the crop markers, just as you would edit a video clip.