They also use all those uglified class names so you can't easily target a class to block. They're total bastards. The Facebook Purity plugin is hip to all these tricks though. There are some very dedicated and talented developers who have put in a lot of time and creativity to circumvent these assholes.
If I inspect the element in Firefox, it’s a div with about 8 layers of randomly named nested divs inside, sometimes branching (but the mouseover border preview tool helps). It takes 10 careful clicks to reveal the final div and its background image URL. Maybe there’s an “Expand all” button, IDK.