How was this done?

[DJH2]

Does anyone have the map that this screenshot was taken on, or know how to make an effect like this?

Bungie.net : Halo 3 File Details

[Huntar]

well, i think the map is Avalanche.
Looks pretty easy, set up a shield wall, Add some Camo, wait till there almost depleted, and take the shot.

[Black Theorem]

The map's Rat's Nest, it's in the description.

There's a room behind default Camo Spawn that is like the Black Room in Snowbound, only better. You need to Turret Glitch into it, go into the right section, and throw down a Flare. It produces that effect.

Just a PROTip: Screenshots can benefit from effects, but it's the ones without effects that are more impressive.