My lock screen wont change

[AC Question]

I just got my lg g4 a day or two ago and i relized that the lock sreen had changed itself without me doing anything to it. It went from the default lock screen to literally the thumbnail of a random youtube video i watched. I dont have a picture or screenshot of the thimbnail either. Now whenever i go into settings>display>lockscreen>wallpaper and try to change it the screen doesnt change even though it says it did. Help