Only you, if you are the one who created/set the wallpaper, or the Administrator, if he/she was the one who set it, of the network can know that.
Get the admin to find it and change it for you.
My network at work has started force feeding everyone the same wallpaper image. Since they started doing this my old wallpaper is still there and shows up when I pull my CAC card or lock my computer. My question is: what is my old wallpaper's file name? I'd like to be able to change that image even if I can't change my wallaper. Thanks for any help.