I think someone asked the same question quite a while back. I'm still not sure if one actually exists, but if there is I'd certainly like to know as well!
The way I accomplished frame-by-frame capture was to play VLC at 25 or 20% and hold down the screencap button (using Gadwin) for as long as I wanted. Literally that was how I did all my caps, then I'd pick out the ones I'd actually post afterwards. I'd end up with 5,000 to 10,000 caps to go through. It took a really looooong time, but it worked best for me so I always stuck to doing it that way.