Many threads suggest manipulating Mojave.heic, but by itself this doesn't do the trick. But, heureca, I did find a way today.
For as long as your disk isn't encrypted – and for as long as you have only one user account – the log in screen picks up whatever wallpaper you set.
However, if your disk is encrypted, you must toggle encryption for image replacements to take affect:
- In /System/Library/Desktop Pictures/, move out or rename Mojave.heic _and_ Mojave.jpg.
- With Preview, create a .heic file form the desired login image. Rename it as Mojave.heic and place it to its original location.
- Turn off FileVault – and restart. You should see your current desktop wallpaper at login now (unless you've got more than one user account.)
- Log in and turn on Filevault – and restart. You sholuld see the desired login image.
- Apparently, you don't need to create a modiefied Mojave.jpg. Yet if you leave the original in its place, login will pick up that JPG copy instead of the HEIST, as if desperately searching for dunes.
- While you run on battery, the change in decryption or encryption won't be affected, just the switch is turned. This alone was suffcient in my case.
BTW, if you chose a brigtly coloured login image, you will notice artefacts around the progress bar once you're past login on the black screen after. Even the default Mojave.heic has got them if you look closely, but they stand out less because of the dark tint on that portion of the image. Hopefully someone figures this out.