In order to put a picture as your background you have to first upload it to your albums and how to upload them to your albums you need to save the picture on your computer. Once you save it go to your albums and click on add picture to album. When you click that you will see 3 spaced where you can upload 3 pictures at a time. WHat you do is click on Browse and find the picture you just saved onto your computer. After that scroll down and click on save changes and there your picture will now be in your album.
Now onto getting it as your background for your profile. Go to user cp and find customize profile. Once you find it click on it and you will see abunch of stuff that says Background, text, border color, etc. What you want to find is Background Image. When you find that click on the little icon next to it and it will bring up your pictures that you have in your album. Just click on the picture that you have in there and the link will pop up in the background image space. After that just click save and there you go, you now have your picture as your background.
Hope that helps O_O I would of provided screen shots instead but I'm on my phone and it doesn't let me do that crap >_>