I think i want to do the same as you and here is a copy/paste of my discovery. That is, the application GRAB will do what we want it to do excellently and it is already on OSX. I cannot believe it.
Hi,
I found VLC and downloaded it, anyway when playing VLC and reading the instructions there is indeed a screenshot capture but in reading this a little more it mentioned the 'GRAB" application on MAC OSX. I went to my app and sure enough it was there and its a breeze to capture excellent quality photographs from movies simply by rolling frame by frame to you arrive at a in good focus shot, then having taken the GRAB icon into the dock open grab, click on capture, click on select and then take your cursor to the top left and as you move it to the right and down a red border appears. Typical had on my computer all the time and i didnt know it. It means i probably wont need VLC really as imovie is better to move backward and forward to get the best focused frame.
Cheers
Maxy