Really based on skinning themes. Usually most pictures are blocked by the osu!cookie so if there multiple menu-backgrounds, if there multiple characters of the skin theme they are using, they can use each character for it own multiple background.
Example. Mirai Nikki skin. A background for each character just to make it more better instead of a one clumped with each character.
Example. Mirai Nikki skin. A background for each character just to make it more better instead of a one clumped with each character.