export video frame

Use the slider above and to the right of the timeline to expand out your timeline. Move your playhead to the frame that you want to export. Click on the export button (the little square with the upward pointing arrow in it) in the upper right of the screen. A export options box will appear.

Click on the Image icon to export your frame. The Image icon may have a different name in operating systems from Catalina forward, but it is easily identifiable.

Another way is to take a screen shot (Command-Shift-4) of the frame as it appears on the preview screen.

-- Rich

Here's what the export options box looks like in Catalina and later:

You would click on Save Current Frame.

-- Rich

To clarify, are you saying that you have a 1 second video in iMovie, with perhaps 30 frames in it, and you want to export one of those frames? If that's not it, would you provide more details?

Also, what versions of operating system and iMovie are you running?

-- Rich