Its easy af. Just do what i write down. “Copyright 201X [Ήєℓℓƒσяcє] This item is not authorized for posting on Steam, except under the Steam account named Ήєℓℓƒσяcє”

Open ":C"
Go On "Program Files"
Open "Steam"
Open "Steamapps"
Open "Common"
Open "Garry's mod"
Open "Garry's mod" once again.
Open "BACKGROUNDS" and remove all pics from there. Then add your own pics. Scroll down to see how it looks like.