Step One:

Go to your Profile, in the upper left corner hover your cursor over "Viewing User Profile" and click the edit button that appears
On mobile, you click the corresponding section to make the edit icon appear.



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Step Two:

In the panel that appears use the slider next to "Use Custom Background Image"



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Explanation

1. Here You Can Choose The Style Which You Want To Use At Your Profile
2. Use This Slider If You Want To Have The Middle Section At Your Profile Transparent
3. Set the Maximum Number Of Friends Shown In Your Top Friends (Maximum Number Of Friends Slots Can Be Purchased)
4. Use The Color Selector To Pick A Solid Color For Your Background
5. Use This Slider If You Want To Use A Custom Picture As Your Profile Background
6. Put The Direct Link To The Picture You Want To Use As Profile Background Into This Box
7. Here You Can Choose The Position Of The Background Image
8. Here You Can Choose The Position Of The Background Image
9. Here You Can Choose The Position Of The Background Image

Note: Dont Forget To Click "Save Settings" When You Are Done