as IPlural says the moderators do the approviing.
In the case of wallpapers, not all wallpapers make it pass moderation, since the moderators try to apply a certain level of quality when it comes to determining whether a wallpaper makes it past approval. This keeps the wallpaper library from being overwhelmed.
In the case of application skins, I think the moderators only try to check if the skin is original and not a rip of someone else's skin.
I think updates to skins may take longer for approval than new skins because the moderators may try to verify why it's being updated. This helps curtail people from "updating" a skin just so that it appears at the top of the updated list, even though the skin really has no real update to it
If I'm wrong on my assumptions, someone please correct me
