If you do not want do download MPEG Streamclip.
The way I make a still from iMovie 11 is to add the clip to a project.
Move the red line over where you want the still to be captured and right click and select 'add freeze frame' to add a 4 second still.
I do this with all the stills that I want to capture and then delete all but the frozen frames.
I then share the project as Quicktime movie and open the mov in Quicktime full screen when the freeze frame is showing I pause Quicktime then click Shift+cmd+4. this allows you to select the part of the picture you want to capture and it saves a copy to the desk top as a .pgn; you do this for each of the pictures, you then have stills of the image as you want them.
I hope this helps.