Zack was posing for you because you were being picky about the pose. You kept telling him to smile more or stand up straighter, and he started to get sick of you.

"You're going to make me late. Hurry up and take the damn picture.”

He rolled his eyes and just bent down, with Rocco putting an arm around him and holding a flimsy peace sign, and opened his mouth. Just before you snapped the picture, Rocco opened his mouth so he could be just like his owner.