Windows Player Classic is a good program to get preview pictures for your videos. You can just type in 'Windows Player Classic' into google and find a bunch of places do download it. I got it from sourceforge.
You don't have to install a program either, it's just an exe that you run. You go to File -> Save Image to save the current paused image as a jpg. Or you can go to File -> Save Thumbnails, and it will save several screen shots in one jpg.
Here's an example of the Thumbnails. I think it is very helpful.
Hope you guys find this helpful.