macOS Sierra has changed login background

We have two users on my MacBook Pro, each with their own desktop wallpaper. These appear as clear unblurred personal images. The login window however is the standard OS Sierra background, and it is unblurred. Well that was the situation until today. All working perfectly.

Now today /library/caches/com.apple.desktop.admin.png (i.e. the OS Sierra login window image) has decided to change itself to the image used by the predominant user's desktop image. So having told OS X that it is stupid and taken all the steps to make the com.apple.desktop.admin.png image correct (i.e. using all of the documented fixes and processes on the net) the image is now set to the Sierra mountain range again, but is BLURRED.

Absolutely none of the techniques on the net will make it unblurred.

Come on Apple. What has your OS done to screw this up? Are you just peeved because I haven't yet upgraded to OS High Sierra???