Weird display on screen before swipe lock Samsung Galaxy s6 edge plus
Looking around, this reddit comment pointed me in the right direction.
Using ES Task Manager I found all apps with a name containing "photo", and all were recently updated.



Photo Collage Editor was the culprit.




So if you don't like random ads forced on you, don't install this app:



I'm sure there's plenty of other apps that do collages, and are made by slightly more polite people.. I'm sure I'll find out when I need to make a collage again, but I'm not using this app ever again.

EDIT
This app, by the same developer ("Zentertain"), is doing the same thing: