Wallpaper Issue

Hey there! Everytime I shut down my MacBook Pro (13-inch, 2017) and turn it on again, that classic desert wallpaper appears, but that only happens on that screen when my password is required to unlock my computer. When I unlock it, my wallpaper comes back normally. I'd like to see my wallpaper on the password screen when I turn on my computer, because my sister has the same Mac with the same model and she faces no issues with that! I'd thank a lot if anyone could help me with it! Thanks.

PS: I'm using macOS Mojave [version 10.14.2]