You've long been able to customize folders in the Mac's Finder, but only with a lot of non-obvious fiddling. Now with macOS ...

This function can be turned on and off in Finder -> View -> Show View Options by checking the box next to Show Preview Icon, but in some cases it may not work even if you have the setting enabled ...

The symbol, which Adobe calls an “icon of transparency,” can be added via Adobe’s photo and video editing platforms like Photoshop or Premiere and eventually Microsoft’s Bing Image Generator.