Shouldn't my Bootstrap theme's image be responsive?

I built a custom WordPress theme with Twitter Bootstrap 3. I followed along with the course on the site called Team Treehouse. I know Bootstrap is a responsive framework but for some reason the images within my WordPress posts aren't behaving responsively.

I have tried adding the img-responsive class via the WordPress image editor. There was no effect.

So, there is something wrong with my theoretical concept of how this should work.

Bootstrap is a responsive framework - I am using a theme built with Bootstrap - therefore - Images should be responsive.

What is missing here? I tried to get through several confusing articles about something called Picturefill and even tried a few plugins as a 'shot in the dark' but am confused because my fundamental understanding is that I don't need anything else to make Bootstrap be responsive. Obviously I am missing something.