Brian Claassen, of March, Cambridgeshire pleaded guilty to assaulting the girl at Lifestyle Portraits Ltd in Swindon town centre in 2016.
Claassen, 40, also admitted possessing 166 indecent images of children.
He was jailed for three years and four months at Portsmouth Crown Court.
Det Con Mark Harris of Wiltshire Police said: "Without the questioning nature of this young girl, who told her mum and dad in the first place, and the help and support her parents have given our investigation team, it's possible Claassen's activities could have gone undetected.
"Instead, their vital evidence has led to a guilty plea and sentence."
A photographer who sexually assaulted a four-year-old girl in his studio and took an indecent photo of her has been jailed.