As Jfike said previously; you can also use a sky, or rain, or snow etc. 2D pic as a "camera filter", or scrim( theatrical set design term) if you make it semi-transparent and position it in front of background objects or terrains. Here I've painted a sky and mist background in PS and imported it as a 2D background. I just added a "marble" to the transparency channel, and put it in front of a background hill terrain.