Try this (and read the whole post first before responding). Download Process Monitor from sysinterals (google it), and then download and install the logon background changer app I mentioned earlier. Start up both of them. Configure Process Monitor to monitor events only from the logon background changer app. Then use the app to change the logon background. Process Monitor should then tell you which files the changer app wrote. One of them should be the file you are looking for.