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Tutorial on how to use an image in your background
If you want to apply a custom background on your profile
You need to click on the customize option in your profile, then on the color icon of Page Background and/or your Module Background you see the option of "Use colors" or "Use images" click the image one and post the link of your image.
Remove the 'hex code' (#) and use url like that; [url ]IMAGE[/url ] (no space) instead in your link. Save it and check the changes from your phone.