Creating Custom Curtain/Lock Images
The default lock image for a computer—whether when using curtain mode or the Lock Screen command—is that of a giant padlock. You can replace this image with one of your own. Simply create the image in whatever image-editing tool you prefer, save it as any QuickTime-compatible image format with the name Lock Screen Picture, and place it in the /Library/Preferences folder at the root level of the client computer’s startup drive. For best results, the image should be sized close to the typical resolution of the display, but not larger.