ok ill do my best and explain. go to your home page and go to "edit theme" which is on the left. go to "start new theme" and then your ready to make your background. to put a picture as your background like mine you'll need the url so go to www.tinypic.com and upload the image. when it has uploaded copy and paste the last code into the box next to where it says "add an image."  then just mess around with it to get the colours right and stuff. thats what i did lol.