Having just finished the personal portfolio myself, I can say that the instructions do say to use responsive design. I had problems, too, thinking that “img-responsive” was supposed to take care of the resizing. I posted here as well.
It turns out you need to use the 12-column grid classes, so wrap each image in its own set of div tags and add the classes col-md-3 or col-sm-6 to them (using the column spans you need for your images).
<div class="col-md-3 col-sm-6"><img src="myImg.png" class="img-responsive" /></div>
And you probably know that all of your body code needs to be inside a div with class=“container” or class=“container-fluid”.