No, it's not broken. Really, it shouldn't give your the option on a fixed breakpoint, because there's no way images can change their size but it does make it easier to transition to a fluid design without having to remember to change them.
To add to David Asch: Although it is not necessary in fixed width breakpoints, it does resize at breakpoints. But as this is unpredictable, it should not be used.
But maybe it helps a little if you do only wire framing for layout purposes and you just want to check quickly how things behave.…