My screen is my business. Or so I thought.
The Problem: Apparently you cannot use any of the standard OS/X screen capture apps, command line tools, or hot keys to capture a frame from a DVD using iDVD. As illustrated, you just get the dreaded checkerboard of no content.
The Solution: Don’t use iDVD. Use VLC. It’s simple, just launch VLC, Open Disc…, press the space bar to pause, use the slider to find the scene, then press ‘e’ to skip frame-by-frame until you find your frame. Then snap away. You can use the built-in VLC video snapshot function (Command-Opt-S) or any other snapshot utility you like. Windows and Linux users can do the same with VLC.