This is what it looks like. You can't see it in this image but when it pops up on pages with avatars, the avatars (not the backgrounds) show up over the window. It never seems to finish loading and if it does, I have already navigated away from the page I'm on because it makes the page completely unreadable.
This has only been happening for a week or so.
Honestly, I'm not sure it's an ad but I figured I'd report the problem anyway.