You can go to one site I have used called

http://backgroundsarchive.com/

Find the wallpaper you want to use and click on it. It will then give you the code for your site. On your site insert the code into an html custom module.

You can also upload any picture you want to photobucket.com

You register with photobucket. It is free though. On photobucket when you find the picture you want to use. I usually select an 800 X 600 or a little bigger size. You will be given several codes. Use the one that says direct link code. Right click on the direct link code. If I'm not mistaken when you right click on the code it automatically saves it to your clip board.

Now go to a background code generator

I use

http://www.myspacegens.com/handler.php?gen=bgcode

copy the image code you have there (remember use the direct link code)

follow the instructions and you will get the code for the wallpaper you want.

finally insert that code in a html custom module on your site

make you a practice test page on your site when you work with this so you don't mess up your site.

On the background code generator

Select Type use background image

Cover area use Entire Profile

In step 2

Copy Image URL (that's the direct link code)

then Position Center-Center

Repeat No repeat( No tile)