Your image tag needs a specific style for the image to active responsive to the framework. Add the style "img-responsive" to your "class" attribute.
Example:
<img src="/images/picture.jpg" class="img-responsive">
In some instances, other classes may have been added to the image tag. Simply append the style to this list. The following is a good example:
<img src="/images/picture.jpg" class="border-on large img-align-left img-responsive">
Also, remember, the image must be sized larger than the area it is intended to fit. The "img-responsive" styling will only shrink an image down to it's parent container and NOT expand an image outwards. Why? Expanding images will dither and degrade in image quality which will not look good. An example illustrating this is found here: http://codepen.io/jstneg/pen/wBoajx
Further instructions may be found at Bootstrap:
http://getbootstrap.com/css/#images