Photo from iMovie on Catalina

Move your playhead to the frame that you want to make into a still image. Then, from the share menu, share it out as an "Image" to your desktop. The image won't be any more hi def than the video clip from which you copied it.

-- Rich

Another way is to take a screen shot of the iMovie preview screen (Command-shift-4, and press drag to select the screen).

-- Rich