Promotional art
From VNDev Wiki
Promotional art is art created for the purpose of promoting your game that is not also used as an in-game asset can be referred to as promotional art. Examples: “countdown to release” images, merch art (ex: posters/prints, t-shirts, dakimakura, keychains; etc), “celebration” images (“Thanks for 1,000 followers!”).
Related concept: Key visual
