First of all press  alt+f2  to run the run application window and in that type gksu nautilus to open root file browser,now navigate to filesystem  then go to user then share and then look for the folder background now just copy the picture you want to set your login screen wallpaper.

After this restart your system ,now once you get upto the default login screen you have to press ctrl+alt+f1 now you got  to enter your credentials (username and password) in this command based  now once you logged in you got to type two following  commands to complete this process  that are

#export DISPLAY=:0.0(0-it is zero  ,press enter )

#sudo -u gdm gnome-control-center(press enter)

after this to return to graphical screen press ctrl+alt+f7  or cm+alt+f7 if you are using mac……

now what this will do is that it will open the control center from where you can change your default the background by navigating to appearance  and then background and then click add and select the wallpaper we had pasted earlier  just select that and click open close that and control center you can see that your default login screen wallpaper is changed.