We need to have reasonable understanding for each case.

Yes, I agree. And in this case it isn't clear that it was inappropriate behavior. This was a group photograph. In group photographs, it's normal to put arms around someone's shoulder.

Now, I would agree that you shouldn't normally do that to someone you have just met today. But maybe the guy was just oblivious and he does this with every person in a group photograph, regardless of the gender of the persons.

Also, they were colleagues, and they were introduced to each other, according to the OP.

Also, according to the OP, it was just a casual hand on the shoulder, so it sounded like a very mild touch.

So all of that are things we should take into account.