The Github wikis are backed by a separate git repository then the main project repository.
This post by the Github team makes it sound like you should be able to link to
For viewers wishing to view images on a github wiki I've found the following method to work:
On your local machine use whichever Git command line tool you wish to cd into your local directory of choice and get the repository via
git clone git@github.com:USER/REPO.wiki.git
Now within this repository create an image directory, I'll call it "images" and put whatever images you want into the directory
You will not be able to physically see the images in say the "Pages" tab, but if you simply reference the images in a local link you should be able to see them. Example with Markdown below:

You may also embed the image as a link itself by wrapping it further like so:
[](http://Yoursite.com)
You can also add subdirectories within your local copy to help organize the wiki since the "pages" tab simply lists them out. Keep in mind, however, that even when the new directory is pushed the "pages" tab doesn't list the directory.