In first step you have to have admin rights:
1.run
regedit tool
2. go to HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background\
set dword key OEMBackground from 0 to 1
3.In path %windir%\system32\oobe\info\
create folder name backgrounds
4. Now put your picture „JPG“ file with size less than 256 kb in this folder and rename it to backgroundDefault.jpg