If you do not have it yet, download the Good Lock app from the Galaxy Store. Open that then look for LockStsr, install that plugin. Then open LockStsr to customize the screen.
Ok so I've looked into the good lock however, it does offer some changes to the AOD not the same. Im good at explaining certain things put here we go....... If you have Always On Display on when the screen goes totally black. And all you see is the time only there is an image that you used to be able to customize via image or gif
Then make one using any kind if image editor (if one has a PC that is far easier). But the Gallery app has an image editor and a studio section that can remake images into just about anything you want. Then when the image you like is ready, open that image and set it as the background on the Lock screen.
I do not know that there "used to be" as like some default image that was native to the phone. Any image in your gallery can be used as-is the wallpaper or editing that image to one's taste and use it that way. Opening an image in the Gallery, there will be three vertical dots on the top right, click on them and then choose "Set as wallpaper" , then choose where to make that the background, even using a modified image.
