Changing your background image.

How to change your background.<o:p></o:p>

Click on Profile, then Customize Profile.

Click on Advanced on the Left column.  Then click on the Paper Clip in the middle.

Click on Choose File to Upload your background image.

After selecting the image. 
Click on the Upload Attachment(s) button.<o:p></o:p>

Now to use the uploaded picture, click on the Tab above that
is titled: Browse Archives.<o:p></o:p>

Click on the file name:

Click Use Image.<o:p></o:p>