Every profile has a picture, a description, and a background. For example this is my background.
All the fruit, and stuff. If you don't have background like that your in the right place. First go to preferences.
Scroll down until you find the black backgrounds currently. The profile background is the one I was showing you, the user card background is if hopscotchers would view your profile.
Press the little picture of the mountain and this should show up.
Ta-Da! Now just take or upload a photo and you have your background! Hope this helps!