Why are you taking the photo of the Mona Lisa? If it's because you just want a nice picture of a famous painting, then no the photo is not art, but rather nice looking photograph of a piece of art. If however you are doing something transformative with the framing or compostion or context of the photograph and using the values imbued in the Mona Lisa to try to make some sort of artistic statement of your own, then yes that photo is art.