Forum:Inserting image in background

How do I inserit an image in the background of my wiki page, I have done this once before but I just can not remember how I did it.

My new wiki page

http://seditionwars.wikia.com/wiki/Sedition_Wars_Wiki

my previous wiki page which I inserted rivited steel plates which I can not remember how I did it.

http://steelmonster.wikia.com/wiki/Steelmonster_Wiki


 * You can use this page to change the background.