Okay Satoshi, A placeholder sprite will essentially be your dummy sprite or mannequin. They can be whatever sprite you like: For example: You want a really cool warlord but no one is willing to make you one so in its place you put Ralph down as the sprite and so on and so forth until the sprite you want comes along and does and makes your sprite. Think of it as the film industry: You hire an actor that you don't really want but at the moment don't have enough to pay for a better one so you have to keep them in for now.
I hope that made sense.
