GreenMartian over @ Neowin Forums has created a little program for automatically changing the Windows 7 logon screen.
This takes all the pain away from doing it manually

Does all this in one go:
* Enables/Disables the registry key
* Resizes the selected image to your current resolution (or any other size if you tell it to)
* Sets it to 96dpi 24bit
* Finds the highest JPG compression that would still keep it below 256kb
* Saves it as backgroundDefault.jpg in the correct folder (and creates a backup if one is already in there)
Source included for the curious (and paranoid. NO MALWARE!) Feel free to do anything with it.
Download:
