You are setting this registry value so that you can see a custom welcome screen background:
However, this registry key resets back to zero after you click, select or create a different theme in the "Personalization" control panel, hence the custom background no longer shows.
When you change the theme in the personalization control panel, it will change OEMBackground to match the value stored in the theme ini file.
Windows 7 has new Group Policy options to control it. Please use the following steps to enable custom welcome screen background:
1. Run "gpedit.msc" in a command window.
2. Navigate to "Computer Configuration" -> "Administrative Templates" -> "System" -> "Logon".
3. Enable the policy "Always use custom logon background".