How do I answer Christians and atheists at work about this?

You can tell them that Iran as a whole is a Shia nation and that they don't follow all of the rules correctly. In Islam we are told to Follow the Quran and the Sunnah, and in this we are told that we shouldn't have any pictures of the Prophet (peace and blessings be upon him) If they choose not to follow that, then Allah will be the judge when their time comes. That's really all there is to say.