I’ve been doing this for years! When someone leaves their user account logged in on a school computer, I change their background to this picture of an obese horse and I re-name it as NowYouWillAlwaysRememberToLogOut.jpg and then log them out.
I’ve never seen anyone leave a computer logged in with an obese horse background, so clearly its pretty effective.
And one time last year I changed a background of a logged in computer and upon logging out I realized that it was ME who had left the computer logged in… I’m a smarty pants!