Pictures as follows:
1.) Pictures of New Mombassa. A human city on Earth being ravaged by the Covenant.
2.) Pictures of the Silent Cartographer. An island with gigantic Forerunner structures built inside and beneath it.
3.) A swamp. Contains a structure that acts as a holding cell to the Flood.
4.) The multiplayer map, Waterworks, which is a piece of art in itself.
EDIT: AMAZING HALO 2 FAN ART
http://www.deviantart.com/view/13441384/