In previous versions of Windows, you need to do file hacking to change the Logon UI background. In Windows 7, it is supported now and can be changed by using the below mentioned Registry entry:
- Key Name: HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background
- Entry Name: Background
- Type: REG_DWORD
- Value: 1 or 0 (1=enabled, 0=disabled)
When Windows 7 starts, it checks the above mentioned registry entry to check if Logon UI is enabled. If it is enabled then the files stored at Windows\system32\oobe\info\backgrounds are loaded at the Alt+Ctrl+Del screen. The file name in this folder must be named backgroundDefault.jpg.