Going forward, you can deal with this by either following up a wallpaper change with a gpupdate /force to all your clients, or by simply making a change in the GPO after a wallpaper change. Personally, I'd recommend changing the GPO - the most obvious way that comes to mind is by changing the filename of the wallpaper each time (add a version number to the end, for example, corpwallpaperv2.png) and updating the GPO to fetch to the new version.