Posterized images on screen

My wife’s Mac Mini just started displaying all onscreen images with a sort of posterized effect. It’s subtle, and it’s not the entire image, just portions of it. I saw it in two different browsers and in the photo app so it’s not just one application acting up. What could be causing this to happen?

The monitor is sort of old so I wonder if it’s starting to go? I’m not sure I have one to swap in but I can probably find a cable to switch to see if that’s causing it.

Any ideas welcomed!