it's under the "main" section, which is the first one :3
"Background Image" should be to the right of "background color"
Also, did you make the album a profile album? You can pick what kind of album type it is when you make it. Only profile album pictures can be used for customizing your profile~
And then click pictures and albums on the left side. Create a "profile" album and upload your background there. Then get its URL from the album and go to "customize profile" and put it in as the background image :3