TuneUp Styler (tuneup utilities 2011) has its own method of adding logon wallpaper. You cant change it via regedit and non of the system files are touched. It creates its OWN authui.dll file called authuitu.dll. Back in the vista days I tried to remove it via the registry. I couldnt. I also went past my 14 day trial, so i was screwed. I had to use system restore to get back to a preinstall of tuneup utilities. The uninstall should have restored the logon, but it didnt.
If you figure out how to remove it, please let me know.