Hi. If U want change only background of the login screen. U can find image in folder. Use "sudo nautilus" (ignore errors)
/usr/share/backgrounds/gdm
Just replace the image and rename to "gdm-wallpaper"
If U want blure effect use gimp to edit 
blure effect lock screen=login screen, I think blure 50 (Filters\Blure\Gauss blure) and brightnes -50
Sometimes resolution in login screen is difrent.
"Log in to your desktop normally and Configure your resolution. Once done, copy your configuration to apply System-Wide:
sudo cp -i .config/monitors.xml /var/lib/gdm/.config/
chown gdm:gdm /var/lib/gdm/.config/monitors.xml
" - source - How to change resolution of the login screen? - #3 by plastronix