The new feature to customize your profile backgrounds is become available now in League of Legends client through the Patch 8.9.
Riot Games has added a new tool that lets users change their profile background to a different champion or skin splash art in game Client.
To change your profile background, simply click on the gear in the top right of your profile in the Client.
Then select your favorite background from your collection.
Then press save.
While Riot gave no additional details on the new feature, it seems from Moobeat’s screen grab of the tool that players will be limited to choosing from the splash arts of champions and skins that they own— only those in the player’s “My Collection” tab appears. This specification seems to indicate that champions and skins not owned will have their splash arts locked from use until purchased by the player.
The new feature is available now in your own Client through the Patch 8.9. So go to there and start changing your profile background as you wish.