Mountain Lion defaults the lock screen background to the last desktop background image that was up before the computer locked. AS, Yet, the only way to influence this behavior in 10.8 is so set a static background image that you don't mind having as your lock screen, or use an app like Lock my Mac
Otherwise, here's how to Replace the Login Screen Wallpaper in Mac OS X Lion/Mountain Lion
- Open the image you’d like to use for the wallpaper in Preview and convert it to PNG format by clicking File -> Export.
- In the dialog box that appears, select
PNG from the Format dropdown list and change the filename so it reads “
- Back up the original wallpaper image by opening a Finder window, hitting Shift+Command+G and typing the following path:
4 .Now copy the
NSTexturedFullScreenBackgroundColor.png file to a safe backup location.
5 . Then click and drag your new image onto the Finder window so it overwrites the original. You’ll be asked to authenticate, so click the button in the dialog box that appears and type your login password when prompted. Choose to replace the original file when prompted.
That’s all that’s required, but you’ll have to reboot your computer for the change to take effect.