John Fisher, 46, admitted touching the girl inappropriately in January 2010 and November 2011, after the photographic evidence was presented, Britain's The Daily Mirror reported Wednesday.
Detective Constable Annabelle Bunn, the Suffolk police officer who worked on the case, said after the sentencing Tuesday, the girl's quick thinking gave police "indisputable proof of what had happened."
"She was brave enough to come forward and report the incident and give us a full and accurate account of what had gone on," Bunn said. "Although Fisher initially denied assaulting her, when confronted with the image he had no choice but to concede he had done exactly as she had reported."
The Ipswich Crown Court ruled Fisher would be placed on the sex offender register indefinitely, the Mirror said.