Nevermind, pretty sure I found it. Try posting again.
"Here's a nice commission I got from Stacktite for the nice price of $60. Unfortunately, he's told me he's uninterested in drawing more true ponies, but he likes drawing humans and anthro, for those of you who like that. I gave him the idea of a first-person perspective with the human viewer having sex with Rarity, and a mirror to show another pose I wanted to see but not without self-inserting, and he gave a great initial sketch on the first try. After that were some minor refinements before he finished it. I like how he had the idea to add sparkles to Rarity here. I don't know if I'd rather think of them as something all unicorns or ponies have, or simply something Rarity would wear, but both interpretations are pleasant."