mauvais
on reddit or something
Rubbish. In any case, the photo is owned by the photographer, who controls the copyright. Simple as that. Therefore taking the kind of action detailed here is appropriate.
The side issue is that the photo contains someone else's work - arguably it is entirely based on it and does not simply coincidentally feature it. Therefore the photographer could be sued by the graffiti artist, should they desire.
Separate issues.
Edit: I believe, perhaps incorrectly, that if you'd simply posted it to document the artwork in question, you could claim fair use. Be very careful if you try to profit from it, financially or otherwise.
The side issue is that the photo contains someone else's work - arguably it is entirely based on it and does not simply coincidentally feature it. Therefore the photographer could be sued by the graffiti artist, should they desire.
Separate issues.
Edit: I believe, perhaps incorrectly, that if you'd simply posted it to document the artwork in question, you could claim fair use. Be very careful if you try to profit from it, financially or otherwise.

