Each racer ships with built-in animated artwork. To replace one: Upload sprite sheet takes a square PNG with a 4 x 4 grid of equal frames (frames 0 to 7 running, 8 to 11 falling, 12 to 15 getting up, character facing right, feet at 94% of the frame height). Upload idle/win sheet is the optional second sheet (0 to 3 idle, 4 to 11 victory, 12 to 15 exhausted). Upload still image takes a single transparent PNG that bobs along the track instead. Preview every animation state.