I can't believe what kind of managers there are out there. Not locking your computer is a pretty small misstake and I doubt its very likely to lead to a real security problem. I think there are much worse things you should concentrate on. If someone unknown sits by my computer and starts stealing stuff I can be sure someone around starts raising some serious questions.
Ok so you forgot to lock the computer, made a misstake. Do you handle all misstakes the same way. Send out on email to the entire company: "Listen up people! Today John has fucked up again! He did this and that! Shame on him!" Like public humilation for small misstakes? I'm pretty sure your company goes straight to hell with that attitude. If it's a real problem, you take it up with him privately.
Changing a screensaver, well thats funny. Public humilation, or straight out sabotage on someone elses computer is not. Not to mention sending email in someone elses name is a criminal act.