I am assuming (possibly incorrectly) that this pretty animated short was made using the oil paint on glass method. This is one way that you can take paintings and slowly alter them to make it appear that pictures are moving. It's a very, very time-consuming and difficult process and I can tell that the film makers of this film worked their butts off to make this. The only reason I didn't score this a lot higher is that several other film makers have recently used this method and have gotten better looking results--with higher frame-rates and therefore smoother animation. With Aleksandr Petrov making films that are CONSTANTLY being nominated for Oscars or winning for his breathtaking animations, it's hard for the makers of LA JEUNE FILLE ET LES NUAGES to compete.
As for the art style, instead of the sentimental and detailed images like Petrov, this one uses a style that is more like the simplicity of the 1930s--almost Edward Hopper-like. It's all very pretty and well worth seeing.