Posts in Jekyll are generated from markdown, so it can be pretty difficult to provide additional classes for styling. You can however take advantage of liquid tags, like include to help you out.

This is how I did it:

1) add a new image.html in your _includes directory


<div class="image-wrapper">
    <img src="{{ include.url }}" alt="{{ include.description }}" />
</div>

2) add styling in one of your css files (I’m no designer)

.image-wrapper{
  max-width:90%;
  height:auto;
  position: relative;
  display:block;
  margin:0 auto;
}

.image-wrapper img{
  max-width:100% !important;
  height:auto;
  display:block;
}

3) whenever you would use the traditional image markdown use the include tag

![Octocat](https://github.global.ssl.fastly.net/images/modules/logos_page/Octocat.png "octocat")

would be

 {% include image.html url="https://github.global.ssl.fastly.net/images/modules/logos_page/Octocat.png" description="octocat" %}

Example (resize browser):

Octocat

octocat