The Gold VIP usergroup allows you to customize your profile. One of the profile customization options is a background image. This tutorial will show you how to set a background image on your profile(if you're Gold VIP).
Step 1:
Go to your profile and click "Customize My Profile".
Step 2:
Click the button next to Page Background.
Step 3:
Click the "use image" button.
Step 4:
Enter a URL or click the Choose Image button to choose an image to upload. You may also adjust the repeat setting for the image.
Step 5:
Click the submit button.
Step 6:
Save the new image by clicking the "Save" button.
Alternatively, there's a short video version available: