help making thumbnals of videos

I have many wmv videos to post and I want to make some thumbnails, I have tried using Screen grabber 2.0.2 and all I get are black boxes. I am using a mac. I even converted the wmv to avi but it crops the move when making the capture (see image)
[URL=http://www.casimages.com][/URL]



anyone have any suggestions?

Over my head but this might be helpfull http://java.sun.com/javase/technologies/desktop/media/jmf/2.1.1/solutions/ScreenGrabber.html

Or perhaps a different software will do it http://www.pure-mac.com/screen.html
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[URL=http://www.casimages.com][/URL]