Home page banner and post page banner is set to be the same by default. You can specify a custom cover in the front-matter:
title: My awesome title
date: 2016-07-25 18:38:45
tags: ["awesome"]
cover: /images/awesome-bg.jpg # <= remember to add this line in your post
---
You will need to create an images folder under your blog’s source folder, and put your own awesome-bg.jpg in it for the above config to work. Of course, you can use whatever folder and image names you want, but be sure to reference them correctly in the cover line.
your-blog
├── _config.yml
├── node_modules
├── package.json
├── public
├── scaffolds
├── source
│ ├── _posts
│ └── images # <= here
└── themes
Also, be aware of the color of the cover image you choose, since the title and menu color are set to white by default. Texts are very hard to distinguish in white backgrounds, although I’ve given the texts some shadow already.