UN officials would not comment on why the wall relief, inspired by Michelangelo's "Creation of Adam", had been masked by a large white screen, referring questions to the Swiss authorities.
But Swiss newspaper Tribune de Geneve claimed that the aim was to avoid offending the Islamic republic's delegation for the talks taking place tomorrow and Wednesday.
Iranian men are expected to cover their arms and legs in public, with women further obliged to cover their hair and wear loose clothing in line with the hardline interpretation of Islam endorsed by powerful clerics in the country.
Swiss officials declined to address the newspaper's claim, telling AFP that the aim was to provide a neutral backdrop at the entrance to the meeting hall.
Although the talks are to take place in the UN's 1930s-era Palace of Nations, the world body is not their official host.
Instead, Switzerland was asked to organise the meeting by the so-called P5+1 group made up of the five permanent UN Security Council members Britain, China, France, Russia and the United States, plus Germany.