I see more often stupid wallpapers on people's phones. I don't want to tell people what should they have as wallpaper, but some people look like they need a special training to do a simple job like this.
Yesterday I was out with a group of people, some of them were my friends, some I've just meet for first time. I could see by accident, because I don't keep my eyes down on the phone like most people do nowadays, I could see what each of them have as wallpaper and after I left, I continued until the end of the day, to ask friends and random people as simple as that, "hello, can i see your lock screen and wallpaper please?"
Most of people were kind enough to show me, but I've meet people who refused. Porn maybe?