Well, first of all, you probably mean MANGA is not art c: Anime is animated.

Your biggest argument might be that Western-style cartoons aren't considered real art (not that I agree with that ;p) and manga is simply an eastern style of cartooning.

For the sake of playing devil's advocate, though, I think this pretty much proves irrefutably that anime = art.

More stylized examples of very artsy anime are here and here.