I think you may need permission for this from the artist, but it may not be hard. It's a beautiful photo. There are times when bloggers and critics use photos of public art and since it's public, there isn't any conflict because it is promoting the artist in the press. I would use it for your portfolio and not worry about it so much because you were documenting art that was indeed in public. I think it would be a courtesy to ask the artist though just to make sure.