Prevent display from sleeping I'm trying to use an auxiliary LCD as a light-pad/bed/table by loading up a 1080p white .PNG in preview and setting fullscreen ON.

The problem is after several (20?) minutes of inactivity I get the Apple screensaver kicking in, and then have to enter my password.

How to prevent this?

Nothing in System Prefs -> Energy Saver works.

http://osxdaily.com/2011/03/21/temporarily-prevent-a-mac-from-sleeping/ suggests pmset noidle but that's also fails.
In addition to the "sleep corner" mentioned in Tetsujin's answer, there is also a small app out there called Caffeine which you can use to prevent the machine (and thus display as well) from sleeping. Note that it will even stop a laptop from sleeping when the lid is shut, so be sure to disable it. There are also alternatives like Amphetamine and Owly that seem to do the same thing, but I've only use Caffeine. All of them hit the same underlying OSX setting, apparently.