There is so much to love about this: Harry's mouth gasping as Snape's hand reaches around. The tie over Harry's shoulder, his bare feet, and the way he's gripping the wall. It's all so erotic.
Obviously, I loved Snape's bum, (because it is fabulously done) but I think my favorite part of this piece is how the buttons on Snape's trousers manage to remain in perfect alignment with the buttons on his boots, crazy wall sex be damned. Just as Snape would expect, I loved that little detail. :D