I'm annoyed by people who leave their factory setting background on their phone
Is there a reason behind this?
I am busy and even I changed mine, I don't like lazy people.
You should not judge people over their preferences. If someone asks you to help them change it they might not know how to and you can help them with out being angry.
I just want to change the wallpaper. It's too bright.
Well, it's your right to change your wallpaper, as well as it is other's rights to leave them on default and I don't think that it is lazy of them to do so specially if they like the default wallpaper. It's their phone and they're the ones using it so you shouldn't bother about it.