How?

It provides a non-smooth surface to induce steam bubbles to form.

I know in chem labs we used to use little chunks of ceramic for test tubes, but those aren't desirable in your tea.

>My microwave has a picture of a cup with a spoon in it, this stops superheating.

This appears to be a microwave setting. I don't see how that would provide a non smooth surface

“My microwave has a picture on it” is what was said; graphic instructions are common.

Ah ok I see now!

Sorry if I was unclear. It's basically this: https://physics.stackexchange.com/questions/234042/should-i-...