Is posting a public Facebook picture of my teacher in a private group wrong?

[SpinRichard32]

Hi, so recently me and a couple of my friends found a picture of our lecturer on his Facebook, we arent friends with him but it was his profile pic and he had the settings as public so anyone can see it, the picture was him striking a somewhat embarrasing pose. Anyway I decided for fun to post this in a private group that consists of about 14 of my friends from college.I left it there for a couple of hours and then after I felt everyone had seen it and had a good laugh I deleted it. When I next came in to college my lecturer asked me to come with him and took me into another room and apparently one of my "friends" told him what i had done and he was pissed off and was telling me off like i had done something terrible or illegal.

Did I really do anything wrong? It was only there a couple of hours and I only shared it with people who I know and trusted just to entertain them and make them laugh. Also my lecturer made the decision to make the photo public.

What is your opinion?