Rainbow Dash and Pinkie Pie are from Hasbro's My Little Pony: Friendship is Magic and this screenshot's taken from Valve's Garry's Mod. It is a gift for Forestvoid24 to help cheer him up.

Pinkie double dares Rainbow to try to make her laugh and she accepts it. She tries many funny faces, including to make a farting sound while puffing her cheeks up, albeit bigger than before. Pinkie is doing her best and struggles not to let out a big laugh.