Here’s a nice trick if you want to post on FaceBook a frame from a movie you’re watching. It uses QuickTime Player, which people tend to avoid because of its limited support for file types. However, installing Perian and Flip4Mac broadens support significantly.
Simply start the movie playing and advance to a suitable scene you’d like to grab. Then pause, and hit Cmd+C. Then open Preview and hit Cmd+N to create a new file with the frame within it. Save as usual.