Or, if you play the video in Quicktime, you can just drag the frame out onto the desktop and it automatically creates a snapshot (in the form of a quicktime snippet or something).
I miss read your question . I know you can do it in iMovie 6. I don't see any options in iMovie 08. You can get a copy of the previous version, then set your pla on the frame you want. Then under the edit option select create still frame and then export it.
Play the video in quicktime and do exactly as the above said but before you grab the screen press space. Then you can select a window to have a screen shot of and if you choose the quicktime one you should have a screen shot like you want.
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