Allow implicit and explicit definition of the default image post

The main image of a blog post is implicitly defined as the first image found in the HTML (see code here), we could make this more explicit in certain scenarios.

My suggestion is to ensure that the image can be identified by a pre-defined class name as follows:
<img class="hero-image" src="/someimge.jpg" >

The code should find this image regardless of where it is in the post and present it as the hero image.