One of these problems concerns the lock screen. Before the Anniversary Update, when you hit a key or swipe up the lock screen to get to the sign-in screen, you had a choice between a solid color and Windows’ default image, and this was changeable through a registry key. However, with the update, Microsoft have changed this a bit, and now you can select your lock screen background to appear in the sign-in screen as well. The thing is, it doesn’t always work.
Enable Windows Animations
First of all, you should have the option of your lock screen background appearing on the sign-in screen too enabled.
Even though this setting might seem completely unconnected to the lock screen, a number of users have confirmed that it does indeed solve the problem. Hopefully Microsoft will address this issue in a later update to the operating system, but until then, following the aforementioned method will help you get your lock screen background image on your sign in screen as well.
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