Upload Images to GitHub's CDN
First, visit any repository on GitHub and click your way through to the issues page.
Create a new issue by clicking the button New Issue. You'll now see both a title and a description field. Drag-and-drop an image on to the description field. This will start the uploading process.
In our example below we're uploading an image of Marty McFly.
Once GitHub is done uploading the image they will automagically add the URL to the description field.
Now you can copy the URL and use it in README files, issues or pull requests how ever you like.