TMPGEnc grabs great screen caps from vid files -- and it's free Aside from being able to copy and paste screen caps into your favorite image proggie, it does a shitload of other things. Definitely worth the download.
To grab screen caps, open the video file in TMPGEnc, then choose the checkbox for "source range".
You'll then see the first frame of the video file, and be able to scroll frame by frame until you find the right place you want to make a screen cap of. Then just right-click on the main preview panel up top, and choose to copy the image to the clipboard.
Then open up photoshop, or any other image editing software and paste the clipboard image in. Voila... there's your screen cap. Now just save it as a jpg or whatever image format you want, and you're done.
http://www.tmpgenc.net/e_main.html