Post in container but images full-width throughout the page

Hello!

How can I achieve having Medium like style where I have my content in one column that is around 700px wide but every image I add to the post will be full-width from edge to edge of the browser window?
The most simple solution I see here is to get rid of theme's post/page layout part where it specify a class (like post-content etc) and include the class directly in Markdown. so I can have something like this:

<div class="post content">
Hello
Hello, it's me
Hello from the outside 
</div>
// include full-with image
<div class="full-image">
<img>   
</div>
// continue post
<div class="post content">
I must've called a thousand times...
</div>

The bottleneck here is I need to include these classes every time I write article so it might slow down my work.
Any other solutions?