Ask HN: Why do icons from Facebook contain ads?

I don't know what is going on, but when I went into my profile, I saw the background was an image.

http://imgur.com/4OwJhgn

Upon inspecting this picture, the url was from facebook.

http://i.imgur.com/xWkPco6.png

when I open the same image in incognito mode, it's what it should be.

http://imgur.com/hmhQWFy

What the hell is going on???