Wow you have been busy with that one! I have looked into imageres.dll before with Microsoft Visual Studio and the wallpapers are there, but you can’t select anywhere to use them. But if you can when using the tokens.dat from a Windows Vista OS it looks like it is Licensing related like the Media Center application and I think like the games and some other Vista applications too.
Maybe it’s a possibility to start Sysinternals Process Monitor, press WindowsKey + L, re-logon, stop the monitor and watch what registry keys and files are read. I don’t know if in Vista the background of the logonscreen is directly changed after for example using Stardock’s LoginStudio, but if it does, it is possible that Windows reads some key to get the background it has to show. (This is just a thought, don’t know if it will work! ;))