Full width images in post

I have a kind of complicated request here.
Basically, I want to display all of the Full size images in my posts to a full width of the <div class=”pad group”>, but to keep everything other than them as it should be (with a padding).

This is how my post looks now: Before
This is how I want my full size images to be: After

<div class=”pad group”> has this CSS code: .pad { padding: 30px 30px 20px; }. When I turn padding off, or set it to 0, everything goes to the edges, which is normal. So, again, my question is what code do I need to use to be able to display only full size images to a full width of the post? Those full size images have this CSS code: .entry img.aligncenter.

PS. I have a local WordPress installation for testing purposes, so don’t mind me copying a post from The Verge.