Well, Sans' hands look a bit too small when compared to his head and the speed lines on Toriel's fist makes it look like she swung around to hit Jerry as opposed to punching outward. I think the background could stand to be a little brighter, too, just to highlight the force behind the hit.
I love the use of foreshortening, though. That's a hard thing to nail down and you seem to have a good grasp on it. Shadows look good and the details on Jerry's face are excellent.
Good work overall. =)

Lol, but where's Papyrus?