We deploy a package which replaces the file you listed with our branding as well. However, it too does not appear with that blurred / frosted overlay, it's just the solid branded image.
Not sure how to fix this either, but just wanted to share that it happens to us as well. Not super critical or necessary for us though.
The image is the last users desktop.
So if you set a company Desktop, & login then out of a Mac with that desktop set. The loginwindow will have the "frosted" looking background.
So if i set my users desktops to our logo, then when machines are locked/booting they will display this image?
I believe this only sets the first time someone logs into the system. So the first users login determines the background. (This includes any fake users for setup or the adobe user for install on reboot items from what I've seen).
Gabe Shackney
Princeton Public Schools