See also: skin.ini
The following are toggles that can be added to .osu (and in some cases, .osb files) to affect skinning / storyboarding. Those that are added/removed by the Beatmap Editor can also be included.
UseSkinSprites: 1
Allows storyboards to refer to the player's current skin's sprites by their filenames. This is pointless if you're already forcing a skin (simply include the needed files) or have a map-specific skin in use. Source