From Frankie's post, we know that it was a picture taken from the camera mode in Halo 3. Much like Forza 2, we can assume that the camera mode uses post-processing effects such as extra anti-aliasing. Forza 2 used 4xAA, HDR and depth of field as post-processing effects in the camera mode for example.
It's good how we can have such modes available to us in the game. Imagine a shootout so intense, you'd want to take pictures of it and share them. Bungie wasn't kidding when they said they wanted to make a media community out of the game. First it was saved replays with customizable cameras, views and angles, now we get a camera mode in the single player. Good stuff.