An array can contain anything. So you can hold all the sprites in an array and keep an index pointing at the current sprite.
var sprites = [sprite1, sprite2, sprite3]
var current_sprite = 0
func move_next():
current_sprite = min(current_sprite + 1, sprites.size() - 1)
return sprites[current_sprite]
func move_prev():
current_sprite = max(current_sprite - 1, 0)
return sprites[current_sprite]