I use a very low-tech method, but it works if you don't need really high-res pics. Set your video hardware acceleration to "None" (this can be done from the Control Panel/Display/Advanced/Troubleshooting window). Then, when you are playing the clip, just hit the "Print Screen" key as soon as the moment you want to capture is playing. Then, paste the desktop image into Photoshop or other image editing software, and play with the contrast until you get it how you want it. You may need to play with this method for a while until you achieve optimum results.