Hey.. you know how to change you start up background for windows 7. like this picture

down here i will teach for easy. just follow..
Hands-on your approach
This method will require you to modify the registry, and if that puts a knot in your stomach, you should move on to the next section.
Step 1: Open the Windows Start menu and enter regedit into the search bar.
Step 2: Right click on HKEY_LOCAL_MACHINE and select Find.
Step 3: Search for OEMBackground, which should be in ComputerHKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionAuthenticationLogonUIBackground. If the key does not exist, add a new DWORD value with the name OEMBackground.
Step 4: Double click on the entry OEMBackground, and change the value from 0 to 1. 

Step 5: Open Windows Explorer and go to the following path: C:\Windows\System32\oobe
Step 6: Create a new folder named info, and open it.
Step 7: Create a new folder within info named backgrounds.
Step 8: Rename your desired wallpaper as backgroundDefault.jpg and place it inside the folder backgrounds (images must be less than 245KB in size).
That should do the trick, now just lock your PC, log off or reboot and enjoy.