Most interpretations of Islam forbid any depiction of Muhammad because it is seen as encouraging idolatry -- one of the biggest sins in the religion.
Al-Maawdah's reference to "ragged clothing" suggests the picture showed the prophet in an insulting way, which would make it particularly offensive to many Muslims. The spokesman did not elaborate on what the picture looked like.
He said the teacher also insulted a student for wearing a head scarf, which she described as "a barrier to knowledge." He declined to reveal the name of the teacher or the university.
A U.S. Embassy spokeswoman said she could not release any information about the case or confirm that an American was involved.
According to al-Maawdah, the case has been referred to court, but no trial date has been set. A Bahraini newspaper reported that the teacher in question has left the country.
In 2006, the publishing in Denmark of cartoons depicting the prophet sparked riots across the Muslim world. And last year, a British schoolteacher was prosecuted and sentenced in Sudan for letting her pupils name a teddy bear Muhammad. The British teacher was later pardoned and allowed to leave Sudan.