How do I export a single frame from iMovie 10.0.3?
In my copy of iMovie (v10.1.6) you can do this easily:

File &gt; Share &gt; Image...


And it will prompt you to save a jpeg of the current frame.

Alternatively export a high quality video ("Theatre" ?) go to the frame you want to capture and select Copy from the Edit menu. Open Preview.app and select File > new From Clipboard and you have your screen capture.