first u should choose the background u wanaa from any site u want like this site
http://www.blinkyou.com/wp.php

or from any search , but it must be big as the screen size 2 be good as a background ....after choosing ur pic upload it by any site for uploading pic like :

www.photobucket.com <<<<<<< the best at all..

and u can upload tha pic by siginig in the site and then shoose the pic from the browse on ur pc and then submit 2 upload the pic, after uploading the pic paste the second code for the pic which named by Tag ..

after that open ( edit style and look ) and then click next and then paste the code in BACKGROUND URL , and then click save and injoy ur new look for the page ...