I wanted to add this according to https://itch.io/t/276900/creating-a-custom-favicon

But after I upload a square image (tried different sizes) it’s only briefly shown on the edit page. After reloading the edit page it is gone again.

Make sure to press Save before leaving the page.

Derp! that button is so easy to miss since there are 5 big red buttons all stacked on top of each other!