What would cause the user's image on mac login to change of its own accord?

No, not a hack. This could happen if the image in question became corrupted, for example. In addition there have always been reports after updates of settings changing randomly for some users. Bottom line, almost assured to be a system glitch. Try reseting the login image via System Settings->Users & Groups-> click your login image to change it.

ELC0524 wrote:

MacBook Pro 13-inch, 2019, running Ventura v13.4.1

Does this mean my computer has been hacked or is it just some kind of system glitch?

While you are at it - Update the computer to the latest stable version 13.5

refer below

About the security content of macOS Ventura 13.5 - Apple Support (CA)

Keep your Mac up to date - Apple Support (CA)

Maybe an anomalous current spike in the battery's circuitry that caused a cascade effect that ultimately caused a non lethal reaction.

Welcome.

Usually good idea to keep machine up to date for various reasons like Security reasons if nothing more than that.