The education department has also issued a showcause notice to Kamlesh Kumar, an art teacher posted in Government Senior School Sainj.
Mr Kumar's vote has been cancelled and he has been withdrawn from poll duty, the official said.
Mr Kumar was assigned election duty under Returning Officer Raghav Sharma and postal ballot was issued to him so that he could cast his vote before reporting to the polling station assigned to him.
While casting his vote, Mr Kumar had taken a selfie in which the serial number of the ballot was clearly visible, Mr Sharma said, adding that the teacher later sent the picture to his friends which went viral on WhatsApp.
Mr Sharma asked the police to register a case under various sections of the Representation of the Peoples Act against the teacher. He said the vote has been cancelled and the teacher was not allowed to go on poll duty.
K D Sharma said a showcause notice was issued to the teacher and further action will be initiated after he filed the reply.
In Mandi, more than 6,000 government employees have been deputed for election duties in the 10 assembly segments and most of them are teachers.
Requesting anonymity, some of them said many employees were seen clicking photos of postal ballots and they used to show them to politicians contesting elections to get polling stations of their choice.
Authorities concerned, however, denied this charge and said no such instances were noticed and action has been taken in Mr Kumar's case.