Try adding this to your stylesheet:

img {
    width: 100%;
}

Update: As an alternative (as pointed out in this answer’s comments by @JasonAller), you could add class="img-responsive" to each img element. This applies max-width: 100%; and height: auto; to the image so that it scales nicely to the parent element. See the Bootstrap docs for more info.

update for bootstrap v4

The classname for bootstrap v4 is img-fluid
Bootstrap v4 docs