I'm tempted to be all enigmatic and leave you to come up with a really clever explanation.
Actually, there isn't really a deep hidden meaning. Next to faces, hands are the most expressive part of the body and so are very useful when you are trying to tell a story without words.
Snape's hand is included because I wanted to amplify the desolation and the final moment of weakness and I didn't trust that I could do that with his face alone. The hand weakly curled shows he has gone beyond fighting for his life.
Narcissa is gesturing towards Harry to balance out the fact that she is looking away from him. Harry's gesture is, like all the cards, partly indebted to the Ryder-Waite deck and the heaven and earth of the Magician. In his earth-bound hand he is holding a the stone, but I think it is too small to really see.
Thanks for taking the time to consider my cards. I'm really glad you like them :D