I had set my own custom background image for the Lock screen in Windows 8.1 setting and it worked normally on my Surface Pro 4 tablet. However, after I installed the Windows 10 Anniversary Update, the Lock screen showed the date and time in the solid black background without any background image. When I brought up the Sign-in screen by hitting a key, the image was there and not broken.
This is a strange issue in Windows 10/8 and become annoying for many users. Turning off the Animate windows function can improve Windows performance, but that also takes effects to disable other options, such as screen dimming indication while taking screenshots, display the selected background image on both Lock Screen and Login Screen. You could try the method to fix Lock Screen Black Background problem after Windows 10 Anniversary Update.
* Press Win + R keys from your keyboard, type sysdm.cpl into Run dialog box, and hit Enter.
Or you can right click on My Computer from your the Desktop, choose the Properties, and click on the Advanced system setting link in the left sidebar,
* From the Advanced tab, under the Performance heading, click on “Settings” button to open the Performance options window.
* Locate the “Animate windows when minimizing and maximizing” option, check its box to enable the feature, and clcik OK button to apply that.
Once you have done that, go to re-enable the lock screen to display picture by this way:
* Tap or click on the Start menu, and select the Setting app to open it.
* Click on the Personalization category, and go to the “Lock screen” section.
* Go to its right side pane, change the “Show lock screen background picture on the sign-in screen” option to On position for turning it on.