On her back.

"OH FUCK ME!!" Penelope screams "FUUUUUUUUCK MEEEEEEEEEEE!"
You feel your balls tighten and cock twitch as she cums hard on her hand,suddenly you pull your raging hard-on out of her ass and shoot your load all over her back.
"OH YES BABY!" she cries looking over her shoulder at you "FUCKING CUM ON ME!"
Load after load arch's out of your tip and sprays all over her waiting flesh,splashing across her skin and into her hair.
"Mmmmmmmmmmmmm" she purrs when you are spent.
"That was fucking hot.So,do we have a deal?"