Go to SteamGridDB[www.steamgriddb.com] and download your preferred assets;

Right click on the game's box art in your library's home, and upload your cover;


(Optional)
Right click on the game's banner in the "Recent Games" shelf, and upload your art;


Right click on the game's banner in its library details page, and upload your background & logo;
(you can change the logo's position by right clicking again)





And it's all done, enjoy!