is it possible to lock the desktop in Windows 7 and have it continue to display your desktop wallpaper rather than the default Win7 locked-workstation screen?
Yes, you can. All you need is a small utility called Logon Changer from tweaks.com. It would allow you to change the logon/lock screen to whatever image you'd like. And you can even test it beforehand, to see where the mandatory buttons fit in.
Try this one which worked for me link