I think the wallpaper folder is not user-writeable. If you want to change the wallpaper to something other than the defaults, you have to browse through all the photos you have on the iPad. I never bothered to sync my iPad with iPhoto (I have photo sync turned off). You might be able to create an event in iPhoto called "my wallpaper" and sync it to the iPad and have all your wallpaper show up there. I just prefer to get my photos onto my iPad by some means other than usb so I have never tried this.