I think it depends on the context of the graffiti. Like gjfalcone said, if it enhances the structure and is visually enticing then yes, it is most likely art. If I'm driving down the highway and there's some random writing ("don't give up" written sloppily on a bridge somehow sticks in my mind) then no matter what it says, it is vandalism. I think it also depends where it is. The building in the photo looks abandoned or blah - perhaps the graffiti was even sponsored graffiti. The city where I lived sponsored local students to basically do graffiti on a parking garage. If these students had gone a block down to the historical market house and done the exact same thing without permission, then it would be a different story.