There are lots of ways to capture single frames from QuickTime videos. You can, for instance, open the movie in QuickTime Player, find the frame you’d like to grab, then press Command-C to copy that frame. Switch to Preview and press Command-N, and you’ll see your copied frame in a new document. You can also use the built-in region capture screen shot tool—press Shift-Command-4, then drag around the frame you’d like to capture. Here’s a third method that takes advantage of Quick Look in 10.5.
First, find the video from which you’d like a screen capture. Select it in Finder, then press the Space Bar to watch the video in Quick Look. Press the Pause button at the bottom of the Quick Look window, then use the timeline slider to find the exact frame you want to capture. Now for the hard part—not! Just click-and-hold somewhere over the image, then drag and drop it on your Desktop (or other open Finder window).
The result will be a Movie Clipping file that will open in QuickTime when double-clicked. From there, you can use Command-C to copy the image and paste it in Preview, or (if you have QuickTime Pro) you can paste it into a new QuickTime document or use File -> Export to save it in another format.
This method may not be any easier than the other solutions, but it’s always good to have alternatives.