Julien de Casabianca started Outings Project to fill the cultural gap between the museum and street. Using nothing but mobile phone people can take photos of abandoned and less-known pictures in museums and transfer them on street walls with the help of Outings team. It is legal until you break a law.
P.s. I bet Julien needs to visit Saint-Petersburg and check the street project done by Russian Museum with the same idea. It was vandalised several if not dozen times until people get used to it, and now you watch Russian Classic Paintings outside.