Update: Github has released Upload option for repositories!
There is an easy way to upload images to Github using prose.io.
But you have to add the code below to _config.yml if you want to upload images to Jekyll blog or if you are using just githb pages then you have to create a _prose.yml file in the root and add this code inside.
This works and this is how I'm updating my blog posts and adding images in it.
prose:
rooturl: '/'
siteurl: 'http://prose.github.io/starter/'
relativeLinks: 'http://prose.github.io/starter/links.jsonp'
media: 'media'
ignore:
- index.md
- _config.yml
- /_layouts
- /_includes
metadata:
_posts:
- name: "layout"
field:
element: "hidden"
value: "blog"
- name: "tags"
field:
element: "multiselect"
label: "Add Tags"
placeholder: "Choose Tags"
options:
- name: "Apples"
value: "apples"
- name: "Bananas"
value: "bananas"
_posts/static:
- name: "layout"
field:
element: "hidden"
value: "page"
- name: "permalink"
field:
element: "text"
label: "Permalink"
value: ""
Now you should be able to upload images through prose.io
Read detailed Tutorial here: How to add or edit Jekyll posts online