Image in post not responsive

I have noticed that some posts have images that don’t resize. Yet some do resize.

I think I have an idea why this is happening but not sure how to fix it.

The ones that do resize don’t have anything in their “CSS Class” field. For example this post: http://hushmagazine.ca/business/12261/#.UuLeLGTTlcw

But this post has “wp-image-3785 alignleft” in the “CSS Class” field: http://hushmagazine.ca/sex/for-the-girl/#.UuLemGTTlcz

As a result (I think), it creates this “div” around the image:
<div id="attachment_3785" style="width: 610px" class="wp-caption alignnone">

When I try to erase the CSS Class field info, it doesn’t remove it all so I can’t seem to get rid of that. In the meantime, I am not sure which WP file the “div” code resides in to fix it. Or is there a way to fix this through the style sheet?

Suggestions appreciated!