Disable automatic changing of wallpaper image

When I set my wallpaper using wallpaper to a directory instead of an image (e.g. wallpaper set ~/.trunk/wallpaper/desktop), it automatically checks the "Change picture every X minutes" checkbox. This does not happen when I set the wallpaper to a specific file (e.g. wallpaper set ~/.trunk/wallpaper/desktop/foggy-trees.jpg). Is this an Apple-imposed limitation? Or could there be a CLI flag that allows me to disable that functionality?