NOTE FOR IMOVIE 10 USERS.
This still works, but from the Project you have to right-click/Reveal In Event. From the Event, you have to FILE/REVEAL IN FINDER. Other than that, this tip should work fine.
First, get an app called MPEG Streamclip, which is free.
Open MPEG Streamclip.
In iMovie, select the clip you need. Then, right-click/Reveal in Finder.
Drag this clip into MPEG Streamclip
In MPEG Streamclip, move the playhead to the frame you want.
In MPEG Streamclip, click FILE/EXPORT FRAME.
Choose JPEG, TIFF, or PNG and give it a name.
You can then drag this photo into iPhoto, Aperture, or the photo app of your choice.
Here is a video I made that steps through this...
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