How do I get a photo out of a QuickTime video?

Pause the video on the frame you want (while paused, you can advance or rewind single frames with the arrow keys), press <Cmd><Shift>4, click and drag to select the desired area with your trackpad or mouse, and the image will appear as a PNG on your Desktop.

When I use the <Cmd><Shift><4> technique, the image it creates appears totally gray. The file is in .mov format. The slideshow plays fine in QuickTime Pro, but the export option is grayed out there. I'm trying to extract a few photos from the slideshow but have been unsuccessful thus far. I'd be grateful to be pointed in the right direction.