Video would be counterproductive to how I implement and tweak animations. I build the animation as a series of repeating pictures in Ren'py, then implement the scene that uses them. Once I see it in action with dialogue, I start to tweak timing and speed until I'm satisfied. Except for some of my earliest animations, the ones you see in LLtP don't have regular intervals between frames. The distance between frames during a sex scene can be shorter when moving in one direction and longer when moving in the other, simulating how thrusting motions work in real life, and I often accelerate and decelerate movement with longer pauses when changing direction. The animations in this game are more complex than they might look, and I prefer to be able to immediately tweak every single detail of them in Ren'py. Besides, most scenes will have each animation play at two (or more) different speeds, and it's not always just a linear change.
Look at the Leanan Sídhe sex scene, where you see her face, for an example of an animation that is extremely fluid in its frame timings. This is particularly true for this one because of the breast physics.
So for me, it's way easier to code these things by using images instead of video. It means I can focus on making the individual images look good when posing, while focusing on the animation itself once the scene is actually working in the game.
