To automate the background is a simple thing. Just Have a batch file [or Powershell script] to create the folder if not empty, dump in the appropriate background picture(s) and use a reg file [or regadd or others] to insert the registry changes. If you don't like the long folder, create a shortcut to it somewhere.
Then when you want to change the background, rename your image file to whatever it is in the reg file and dump it into the folder, overwriting the old one.
[Played a simple prank on someone by inserting a less than nice picture of the guy for his start-up image!]
That said, you see it on startup [if not using a password at start-up] and shut-down for maybe 30 seconds. Is there really a point to using it?