For viewers wishing to view images on a github wiki I've found the following method to work:
Go to the "Git Access" tab inside the wiki page of your repository and retrieve the SSH path which should be some thing like: git@github.com:USER/REPO.wiki.git where USER is your account name and REPO is the repository name.
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
And then push your wiki git directory to github
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:
![Alt attribute text Here](images/YOURIMAGE.ext)
You may also embed the image as a link itself by wrapping it further like so:
[![Alt attribute text Here](images/YOURIMAGE.ext)](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.
I put both images and PDFs in my wikis. I clone the wiki and add an images and a files directory, then use the following markdown to embed image links and add file links:
Images:
[[/images/path/to/image.ext|ALT TEXT]]
The leading slash isn't necessary if your wiki pages are all at the root level, but I use subdirectories and an absolute path resolves correctly and keeps things simple.
Files:
[link text](files/path/to/file.ext "ALT TEXT")
Note, no leading slash for the wiki files path to resolve correctly as a link in this format.
I documented this with more detail in a GitHub gist