Good app / process for organizing video snapshots into a storyboard?
I ended up using Trello.  It was nice in that I can lay out shots scene by scene, with each scene as a list.  Dragging and dropping individual frames (as png's or jpegs) for card cover images was super handy, as was being able to move around and organize the cards later on.

Being able to write longer descriptions and comment on each card allowed me to document why I wanted to use a shot, or decided against it.