All images are supposed to be responsive as you can see on the Epik demo….But your code has a fixed width added inline (not from the theme, or css) –
<div id="attachment_112735" class="wp-caption aligncenter" style="width: 633px"><!--formatted-->
Anytime there is a fixed width added inline, it won’t be responsive. So try and remove that width from your code and it should work.
