How could the wallpaper change spontaneously in Android?

Question

Malware

Last night I turned on the alarm clock, this morning I woke up, there is a new wallpaper on the phone. Really boring. Some guy in a hood, looking like a Sith. The picture that I see for the first time and it was not on the phone and now it is not found in any folders. Nobody touched the phone at night for sure. I'm not a somnambulist, that's for sure.
The first thought is not a very good joke of the developers of the launcher (Nova) on Halloween.
Confirmation not found (look at w3bsit3-dns.com, the launcher's website, just Google search for knocks).
The second thought is a virus. Checked by Avastom, Dr. Web, Kaspersky. Nobody found anything. A Google search for queries like "virus changed wallpaper + andoid" also did not give anything for a day or for all the time.
I searched for a similar picture in TinEye, the result is zero.
Smartphone LG G2
Andoid 4.4.2
Not root
Shell Nova Louncher Prime latest version.
PS The question is certainly idiotic, but still. Either I missed some banal answer, or this is really a terrible curse.
Picture. Photo because the screen did not work