Most likely you have set the album where the background image is located as "Private".
In general, the following guidelines might be of use:
- Upload the background image (that you wish to use) into your album. Make sure the album is "Public" or "Profile" (it cannot be not "Private")
- In the customize profile page under the box "Main" you can notice "Background Image" with this icon click on it and your image will show up.
- Double click the image and choose the option for "Repeat" and Save.